Contents of the section:

Special glassy materials and their properties

SOME ASPECTS OF COMPOSITE INORGANIC POLYSIALATES J. Šesták, B. Foller J Therm Anal Calorim (2012) 108:511–517 CAN CLUSTERING OF LIQUID WATER AND THERMAL ANALYSIS BEcOF ASSISTANCE FOR BETTER UNDERSTANDING OF BIOLOGICAL GERMPLASM EXPOSED TO ULTRA-LOW TEMPERATURESC. J. Šesták, J. Zámečník Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 2, PROPERTIES OF SOME NATURAL GLASSES: AUSTRALIAN OPALS AND CZECH TEKTITE MOLDAVITESBořivoj Paul Thomas, Klaus Heide, Jaroslav Šesták, Ekkerhard Fluaglain, Peter Simon Book chapter THERMAL PROPERTIES OF AUSTRALIAN SEDIMENTARY OPALS AND CZECH MOLDAVITES Paul Thomas, Jaroslav Šesták, Klaus Heide, Ekkerhard Fluaglain, Peter Simon J Therm Anal Calorim (2012) 110:997–1004 ON THE CHRONICLE OF HIGH-Tc OXIDE SUPERCONDUCTORS Interdisciplinary of thermochemistry, stoichiometry, glassformability and thermodynamics Jaroslav Šesták, Pavel Lipavský Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 365–373

Some aspects of composite inorganic polysialates

Jaroslav Sestak • Bronislav Foller

3rd Joint Czech-Hungarian-Polish-Slovak Thermoanalytical Conference Special Chapter

� Akademiai Kiado, Budapest, Hungary 2011

Abstract The state-of-art of polyaluminosialates is

reviewed in terms of inorganic hmersi showing the com-

position, degree of netting, function of modifying atoms

and the role of non-bridging oxygen as well as hydroxyl

groups (biocompatibility). The polymeric condensation is

compared with the vitrification of glasses upon cooling.

The replacement of Si by P is discussed as well as the

analogous precipitation process of amorphous hydrous

silica (opal). Progress of geopolymers and biopolymers

usefulness is shown within the framework of generalized

world of macromolecules screening hundred contemporary

citations. Pultrusion technology is presented, capable to

produce composite geopolymers reinforced by basalt fibers

staying suitable for mechanical applications.

Keywords Geopolymers � Biopolymer � Polysialate �Polyphosphate � Opal � Non-bridging oxygen � Hydroxyl

group � Pultrusion technology

Reviewing geopolymers and their links

with the classical state of quenched glasses

As a matter of curiosity the novel so-called geopolymeric

materials were introduced about 10 years ago on the pages

of this journal [1, 2] by Davidovits (cf. Fig. 1).

Geopolymers—X-ray-amorphous inorganic polysialates

[1–5] are cementitious composites, which are commonly

produced by idiosyncratic wet copolymerization (i.e.,

synthesis via solution) of the individual alumina and silica

components [1–14]. Such a room-temperature synthesis

process takes place when aluminosilicate source materials

are dissolved in aqueous solution at very high pH yielding

thus mostly non-crystalline zeolitic-like precursor, which is

also termed as copolymer. The reactive Si–OH-based easy-

soluble monomers in water-glass rather easily penetrate

into the structure of inserted solid Al-compounds. This can

be compared with the case of a high-temperature melting

process of precursor glass batch, where conversely the rigid

structure of quartz sand is diffusion-disintegrated by the

penetration of Ca, Na, Al atoms from their more easily

melted compounds thus braking up the original –Si–O–Si–

structure. Both routes factually represent a mutually com-

parable type of vitrification reactions equally facilitating

ionic interactions by either the movable hydrated ions or

the diffusible atoms in the melted viscous state.

Aluminosilicate gels (zeolite precursors) are mostly

synthesized [14–23] with the composition characterized by

general formula Mm[–(Si–O)z–Al–O]n�wH2O, where Mm are

modifying cations (mostly Na, K, Ca, Mg), n is the degree of

polycondensation and z is structural ranking (1, 2, 3,…).

Configurationally tetrahedrons SiO4 and AlO4 are mutually

bonded by oxygen bridges forming thus Si–O–Al-based

chains and rings. The positively charged Mm ions ought to be

compensated by the negative charge of four coordinated Al.

Generated gel-like structure is partially amorphous or nano-

crystalline depending on both the amount of initial solid

matter and its nature (character of raw materials) as well as

on the condition of the reaction conditions (pH). The

amorphous state is primarily favored for a higher concen-

tration of solid precursor in the preparatory suspension.

The gel subsequently hardens into rigid geopolymers

(resembling the glass formation upon the melt solidifica-

tion), which may be characterized in a number of ways,

J. Sestak (&) � B. Foller

NTC—New Technology, Research Centre in the Westbohemian

Region, West Bohemian University, Universitnı 8,

30114 Pilsen, Czech Republic

e-mail: sestak@fzu.cz

123

J Therm Anal Calorim (2012) 108:511–517

DOI 10.1007/s10973-011-2037-0

correspondingly applicable to classical glasses. For exam-

ple, the description in terms of principal constituents

(alumina and silica) [22–26], their structure (tetrahedral

Al–O and Si–O units in random three-dimensional frame-

work), charge-balancing role of the tuning metallic (often

alkali) ions, thermal glass-formation characteristics [27,

28] and their macroscopic properties (moderately strong

and hard, stable up to 1000 �C, etc.). The specific cir-

cumstances of the low temperature synthesis [14–28] such

as the condensation temperature of alumina and silica

resources at high pH and distinctiveness of various sorts of

water-glasses [29] are worth of a further clarification. XRD

patterns of commercial melted glass and wet synthesized

inorganic polymer were compared [29] showing broad

peaks between 17� and 34� 2h (typical for amorphous

phases) which for inorganic polymer is slightly shifted to

the higher 2h revealing questioningly its relatively more

‘‘dense’’ structure than that for analogous commercial

glass. Further deconvolution of such broad peaks in XRD

patterns can enable a more quantitative estimation of the

degree of amorphicity.

Certain formalism was developed in order to investigate

structural units involved, mostly in the terms of fragments

such as [–Si–O–Al–O–] called sialate units [1–14] (or

polysialate when condensed concurrently). Further suggested

units contain different Si:Al ratios, such as [–Si–O–Al–O–Si–

O–] (sialate-siloxo [1–4, 8, 14–16]) and [–Si–O–Al–O–Si–O–

Si–O–] (sialate-disiloxo). The Si:Al atomic ratio implies 1, 2,

and 3, however, non-integer ratios intermediate between 1:1

and 1:3 may be anticipated as changeable combinations of

basic units, provided that the content of charge-balancing

cations is appropriate (often water content controlled). The

units with Si:Al [ 3 are designated as sialate and polysialate

geopolymers. In the sense of majority of the Earth’s crust,

which is composed of siloxo-sialates and sialates, the common

feldspar series are albite–anorthite (NaAlSi3O8–CaAl2Si2O8)

describable as poly(sialate-disiloxo) for albite to poly(disia-

late) for anorthite.

Hypocrystalline materials and their ‘mers’ framework

So far not enough attention has been paid to the basic

structural disposition though detailed studies on reaction

mechanism are available [30–39]. The inherent stoichiom-

etry can be understood analogously to organic and/or inor-

ganic –mers (known in classical polymeric chains) [39–47].

Such generalized spheres of the so-called hypocrystalline

materials (an newly coined term more appropriate for an

approved terminology) are specific of possessing the regular

polyhedrons AOn (such as SiO4, AlO3, AlO4, BO4, BO3,

PO4, etc.) [33, 47]. Important is the linking function of

bridges formed by all n atoms of oxygen. If oxygen is

bridged with two central cations, A- (e.g., {SiO4/2} as AOn/2)

then i atoms of oxygen become bridged with A by a double

bond (the so called terminal bonds, known from a single

component glass composed, for example, of phosphorus

oxides containing –mers of {O=P(–O–)3}. The adequate

coordination formula ensues as {AOi/1O(n-i)/2}, where

{PO1/1O3/2} may serve as an illustrative example of coin-

ciding multifaceted stoichiometry. In the analogy with

organic polymers, the group {AOi/1O(n-i)/2} can be consid-

ered as (n-i)-functional—mer. Similar attitude can be

applied to geopolymers based on aluminosilicates composed

of tetrahedral alumina and silica units. When condensed at

ambient temperature the mer—units of AOn/2 become re-

positionable because of their concoction long lifetime this is

comparable with the degree of immovability at the high

temperature state of melts. Similarly to oxide glasses of a

randomly interconnected web (continuous random network

[40–47]) a relatively more complex copolymer system

(containing supplements of moderating electropositive ele-

ments typically alkaline oxides, M2O, or other metallic

oxides) persevere the function of modifying oxides. In oxide

systems (melts/glasses/macromolecules) such additions to a

single-component (tetragonally netted) solution result in the

breakdown of bridging bonds A–O–A and formation of the

so-called non-bridging oxygen. They can be described by set

of equations: M2O ) 2 M? ? O2- and M0O ) M02? ?

O2- and A–O–A ? O2- ) 2A–O-, which are factually

responsible for the dissociation concerning a single mole-

cule of modifying oxide and the subsequent breakdown of

bridging bonds (A–O–A). In the case of typical modifying

oxides the equilibrium [42, 46] shifts toward the products

and the arrangement can be easily derivable from melt/glass/

polymer initial stoichiometry.

The cation distribution affects the species in the silicate

solution, i.e., the amount of monomers, dimers, etc. [19, 29,

32, 33, 48–50]. Water-glass enforced by few percent of

Fig. 1 Animated discussion between two pioneers of geopolymeric

materials, French professor Joseph Davidovits and Czech professor

Jirı Brandstetr (VUT Brno, 2007)

512 J. Sestak, B. Foller

123

Al-ions [50] or partly substituted by P2O5 increases its

penetrating reactivity. The study of vitrification of silicate

solution indicates that the reaction commencement becomes

associated with a concentration decrement as a result of

separate phase growth [29, 30]. Comparing the molecular

structure of reactants (metakaolinite and silicate solution)

with the geopolymers, a complete rearrangement of

molecular environment (short-range order of Al and Si) is

evident [33]. Alumina is likely altered from their distorted

crystal order via certain intermediate species (associated

with valences IV, V, and VI) and linked to one SiO4 unit in

metakaolinite forming the regular AlIV{Q4(4Si)} [33, 44–

46] where Q’s are the standard representation of Si-based

configurationally motives [44–46], see below. However,

the Al atoms seem to attain a more symmetrical environ-

ment in final geopolymers than in the original metakaoli-

nite. The most reactive species in silicate solution is likely

the hydroxyl anion OH- (or H3SiO4-), which can draw

parallel thoughts toward the basis of bioactivity of inor-

ganic materials [51–56] and the coupled effect of non-

bridging oxygen [47, 54]. It may even catch the attention of

an innovative conception toward life creation on the Earth

[51, 52] providing geopolymers as a new target biomaterial

[57].

Accounting for the specificity of geopolymers, the

charge-balancing metal ions factually make feasible the

crucial polymerization in (–Si–O–Al–O–Si–O–) sialate-

silixo chains where the atomic ratio remains Si:Al = 1:1

(however, capable to increase up to 3:1). When reaching

the value above 3, generalized polysilicates became com-

parable to melted (counter-partner) glass. This approach,

however, has not been applied to a geopolymeric state due

to yet unacquainted methodology. The difference between

melted glass and condensed sialate-silixo polymers endures

in the subsistence of a certain coordination of (–Si–O–Al–

O–Si–O–) with both Si4? and Al3? cations in the fourfold

coordination, which is unavoidably balanced by the pres-

ence of modifying cations. It is ranging from fully amor-

phous up to partly organized (modulated) compositions of

hyper-crystalline states thus exhibiting definite nano-crys-

talline regions. Single and multiple Al–O–Al bridging

cannot carry on to subsist, nevertheless, alternatively can

survive in minerals such as above-mentioned albite

(NaAlSi3O8) and anortite (CaAl2Si2O8).

Worth another attention is still an apparent analogy with

generalized (even organic) polymers mentioning the so-

called mean degree of netting. Structural motives, Q, which

appear throughout the melt–glass–polymers [33, 44, 45,

47] can be characterized by a coordination formula

{AO(i?j)/1O(n-i-j)/2}, where j is the number of bridging

oxygen atoms associated with a single central atom being

ripped away by the action of oxygen produced through the

dissociation of modifying oxides (j B n-i). In the case of

silica glasses (i = 0, n = 4) there is a direct link between

the concept of the so called Q-notation (i.e., Qk, k = 0, 1,

2, 3, 4) and a coordination formula, Qk : {SiO(4-k)/1Ok/2}.

Similar approach may become welcome for its alternative

implementation to the circumstances of geopolymers,

which fashion is still under prospect. Nonetheless, it was

shown from NMR data [58, 59] that Q4Si(2Al) and

Q4Si(3Al) components can exist in alkaline geopolymers,

the latter being the highest for the Si/Al ratio of 1.5. For

higher concentration of Na and consequently lower Si/Na

ratio in the activating water-glass (and with Si/Al ratio

measured in the polymer support) a feasible scheme points

up that the hydrated sodium aluminosilicates bears a three-

dimensional structure in which Q4Si(3Al) predominates.

Some prospective studies

An alternative attitude to geopolymers can be anticipated

when assuming a wider stoichiometry range by, e.g., the

incorporating phosphates in addition or even instead sili-

cates (where Si is totally or partially replaced by P) and/or

borates as an alternative to aluminates. On the contrary to

the needed alkaline environment for polyaluminosialates

the phosphates are formed by an acid–base reaction

between a metal oxide and an acid phosphate. Virtually any

divalent or trivalent oxide that is sparingly soluble may be

used to form these phosphate geopolymers. Berlinite

(AlPO4) may serve as a good example, which is formed by

the reaction between alumina and phosphoric acid

(Al2O3 ? 2H3PO4 ) 2AlPO4 ? 3H2O). It was also dem-

onstrated that phosphate geopolymers of trivalent oxides

such as Fe2O3 and Mn2O3 may possibly be produced by the

oxide reduction and then acid–base reaction of the reduced

oxide with phosphoric acid. Such wide-ranging phosphate

materials represent another variety of mineral geopolymers

[60–63] possibly promoting the incorporation of Fe cations

for magnetic applications [48, 63, 82].

Relevance of bridging and non-bridging oxygen [45–47] to

polysialate–polyphosphate geopolymers overlapping tetra-

hedral alumina/silicate/phosphate units has not been so far

studied nor analysed so that this would become a target of

future investigations. Notwithstanding the addition of acids,

which occasionally accelerates the formation of gels, may

support an idea that the gelling mechanism involves a cross-

linking of preexisting (often linear) polymers someway

analogous to organic polymeric systems. A range of other

activation processes were also investigated [63, 64], however,

any indistinct integration of theory of organic polymers,

occasionally applied even to soluble silicates, may not assist a

better interpretation of such aqueous inorganic systems.

Strategy for geopolymers applicability as a matrix for

reinforced composites (fibers, foam, and/or ceramic) is

Some aspects of composite inorganic polysialates 513

123

different from the practice in similarly activated cements

[25, 65] producing, however, analogous materials that

possesses ‘cementitious’ property. It is foreseeable that the

structures of these alkali-activated cement, alkali-activated

slag, fly ash, etc., are different resulting from different

chemical-mechanistic paths. The calcium silicate hydrate is

a major binding phase in Portland cements holding also

somewhat unstable ettringite. In dissimilarity the binding

property of geopolymers results from the formation of a

three-dimensional (mostly amorphous) aluminosilicate

network containing tetrahedral unit with diminishing

transport properties (causing thus a low mobility of parti-

cles). Therefore, the viscosity of such dispersal heteroge-

neous system tolerates systematical formation of a required

shape by slow rearrangement. The rates of partial poly-

merization reactions are in agreement with the rate of

structure ordering processes. Depolymerization requires

the formation of zeolite nuclei within a geopolymer micelle

in the hydrous aluminosilicate gel. The time needed for

such crystallization varies from a few hours to several

days; aging time at room temperature is about one day and

crystallization time at 100 �C up to 100 h (for the relevant

glassy Na2O/K2O–Al2O3–SiO2 system the crystallization

temperature is in the range from 150 to 230 �C). The

structure of amorphous silica is of a more open arrange-

ment than that of closely related cristobalite. Spaces/sites

on the reaction surface are worth of extra attention being

capable to accommodate hydroxyl ions. Such a reactive

surface bears an ionic charge and particularly silica is

under continual switching process of equilibration between

solution and interfaces, which thus enables better physical–

chemical interaction even with the enforced fillers.

Some practical aspects of geopolymeric composites can

be seen from the matrix primary properties exhibiting the

compressive strength of 100 MPa, which is about twice that

of cement but having only one-half its density. Filling with

all-length polypropylene fibers increases its strength up to

about 200 MPa. For a plausible technological application

we brought in a special so-called pultrusion technology [66]

in which the enforcing material is a yarn bundle, better a

rowing of basalt fibers [67, 68] (2520 TEX, / = 13 lm),

which is enforced through an extrusion nozzle to harden,

see Fig. 2. For a weight ratio of rowing of about 80%, with

commercial matrix (Baucis FG), the resulting banding

strength reaches 360 MPa, which becomes reliable for

mechanical applications (when expensive classical epoxy

analogous composite is not more than twice stronger)

[67–69]. The advantage of this geopolymeric composite is

its excellent fire resistance (so-called fire–smoke-toxicity).

There are some unclear points as the determinability of

glass transition temperature of polysialates. It is often

accomplished from the position of loss modulus apex

(resolved by DMA) [39] but for many amorphous materials

the glass transition temperature becomes overlapped by

crystallization upshots [70]. Better solid phase identifica-

tion is needed [70–74]. So far there were no attempts to

look for a glass-formation coefficient, common in other

types of glassy materials [75], which may provide a better

insight to the structure of geopolymers (and their modeling

challenge [76–79]) as well as for some new applicability

[80, 81].

Further interrelation aspects of far-reaching polymeric

materials

Another extension of a generalized understanding of inor-

ganic polymeric materials can be located in the sphere of

amorphous hydrous silica—opals [82–88], which is a

remarkable material with the general formula SiO2�nH2O,

but is precipitated as mono-dispersed colloidal particles

(from 150 to 400 nm in diameter). In the most common

weathering model [83–85, 87, 88] it is formed analogously

to the gel process [67, 68] via geopolymers-like formation

Fig. 2 Newly employed pultrusion technology showing from the left

yet dry rowing of basalt fibers, in the middle the rowing which is

already dipped to the suspension matrix (kaolin Baucis FC; water

glass Bindzil) finally hardened in the furnace under tension to form

final oblong chips about 20 9 20 mm

514 J. Sestak, B. Foller

123

through gelatinous state of approximate stoichiometry

(Mm(SiO2)Z (AlO2)Y nMOH�wH2O). Sandstones are a

common source of silica where the process of chemical

weathering (of relatively soluble silicates such as the

feldspars contained in these sediments) results in the for-

mation of an alkaline silica solution. An example of such a

mineral is potassium feldspar which weathers through the

idealized stoichiometry (e.g., 2KAlSi3O8 ? 3H2O )Al2Si2O5(OH)4 ? 2KOH ? 4SiO2) by the permeation of

ground water through the sediments resulting in kaolin,

dissolved silica, and an increase in pH through the release

of potassium hydroxide.

The process of cross-linking via sol–gel processes is

characteristic of possible incorporating a substantial amount

of organic compounds as additives [88]. Thus, the charac-

teristic property of a more broadly viewed naturally occur-

ring geopolymers can involve a substantial amount of

organic/humid materials. The term geopolymers can thus be

understood by geochemical communities as macromolecu-

lar organic-containing polymers (or biopolymers) indicating

the geological transformation of various geo-molecules

through geochemical processes [89–93] during diagenesis.

Consequently, such a mineralization path provides a stable

material as the final (alternating) products in the Earth, such

as kerogen, asphalt, etc. Such an attitude is also cross-

boundary to an assortment of geo- and bio-polymers along

with a generalized comprehension of existence of various

biomaterials (for example biological glasses [94–97]) rep-

resenting innumerable organic macromolecules accountable

for life, their nucleation and transformation records, where

an important role plays common states of low ordering,

either forming glasses, amorphous matter, modulated, and

nano-crystalline structures [98]. It incorporates basic ques-

tions of the determinability of such glassy and amorphous

states [70–75, 98], adequate structure analysis [76–78, 99],

search for a wide-ranging material applicability [11, 61, 99],

etc. It involves a huge sphere of biological life, its diverse-

ness, and also its curiosity to ever appear [51, 52].

Acknowledgements This study has been carried out by NTC ZCU

Pilsen under the support of the Project No. FR-TI 1/335 ‘‘Geopoly-

meric composites with high technical parameters’’ provided by the

Ministry of Industry and Business of the Czech Republic and within

the CENTEM Project, No. CZ.1.05/2.1.00/03.0088 that is co-funded

from the ERDF within the OP RDI program of the Ministry of

Education, Youth and Sports.

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Some aspects of composite inorganic polysialates 517

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Introduction

The concise form of NATAS (Bowling Green,USA’06), ESTAC (Cracow, Poland’06) and JCCTA(Kyoto, Japan’06) lectures about the plant thermalbehavior at lowered temperatures is presentedherewith showing another example of a widerapplicability of thermal studies [1]. The method canbe found and conveniently used for the charac-terization of detrimental influence of man-affectedenvironment assuming its injurious impact on plantsgrowth [2–5]. In some cases, the hostile environ-mental conditions (such as a volcano explosion,catastrophic impact of a meteorite, global atomic waror spread out of deadly infections) may even lead to aserious depression, and in limit, extinction of thepopulation density of certain plant specimens on ourplanet. On the other hand, there are also many plantspecies close to the edge of their disappearance underthe currently deterioration effect of changingenvironment or even under a routine fatality ofspecies vanishing. In the sphere of the present vastprogress in genomics and the ongoing growth ofgenetically affected plant there is a supplementarycall for the exigency to store plants with their entiregenetic information preferably using the originalwild-plant sources?

Fortunately, it is not necessary to store a wholeplant but it is enough to have its buds or even anymeristematic cell or tissue because the plant cells aretotipotent; therefore it is possible to store only a small

part of plants, mostly a part of meristematic tissues ofvegetatively propagated plants [4] or a seed axis ofgeneratively propagated plants. The plant storage atlow or even at ultra low temperatures (cryopreser-vation) is one of the appropriate ways how to keeptheir viability in a long term prospect.

However, the intracellular ice-crystal formation,which habitually occurs during the lower temperatureexposure of plants, is in every case lethal for plantsurvival because of the inherent volume expansion ofnewly formed ice in comparison with the originalvolume of liquid. It grants a consequent destruction ofinterfacial cell membranes, sometimes happeningeven on reheating. One way how plants can withstandthe low temperatures in the nature is their tolerance tomovement of intracellular water to extra-cellulardomains, which, in result, bestows a higher toleranceto cell dehydration [4, 5]. However, the undercooledstates possess frequently no dehydration effectsneither ice-crystal formation, but this state isnoticeably unstable providing somehow not fully un-predictable processes of ice recrystallization uponreheating [4]. The exceedingly low viscosity atrapidly cooled conditions of ‘freeze-in’ states giveoften birth to the new non-crystalline state of rigidglasses and such a process of vitrification is notassociated with any volume changes exhibiting onlythe so-called glass transformation region detectableby thermometric measurements. The freeze-in statesnearly hinder any diffusion of molecules excluding

1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary

© 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands

Journal of Thermal Analysis and Calorimetry, Vol. 88 (2007) 2, x–x

CAN CLUSTERING OF LIQUID WATER AND THERMAL ANALYSIS BEOF ASSISTANCE FOR BETTER UNDERSTANDING OF BIOLOGICALGERMPLASM EXPOSED TO ULTRA-LOW TEMPERATURES

J. �esták1* and J. Záme�ník2

1Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Praha 6 , Czech Republic2Research Institute of Crop Production, Drnovská 507, 16106 Praha 6, Czech Republic

Plausible effect of clustering of undercooled liquid water (pentamer configuration, icosahedra formation) is discussed showingwater continuous but non-periodic patterns and its impact to the either formation of ice-crystals or ice-glass, particularly whenmaking contact with plants. These processes are viable for the cryogenic storage of biological germplasm and subject forthermoanalytical studies aimed to the determination of glass transition temperatures.

Keywords: cryogenics, glass transition temperature, icosahedron, natural freezing, non-periodic continuum models, pentamers,plants, thermal analysis, undercooling, water clusters

* Author for correspondence: sestak@fzu.cz

thus biochemical transforms and, accordingly, alsoany genetic changes.

The formation of glassy state is yet not fullyunderstood but expedient to help a successful storageof plants at low temperature of liquid nitrogen. Fromthe point of view of cryopreservation, the glassystate [6, 7], which possesses the same volume asliquid, occurs as the most suitable state for a long-term biological storage of germplasm. This strategyof “being in the glassy state’, is repeatedly exploitedby plants in the nature in order to withstandunfavorable freezing conditions of surrounding en-vironment, which can be seen as a sort of miracle. Forexample, plant pollen flying in the air has a potentialto be in glassy state keeping its viability after stigmapollination. Dormant poplar twigs can survive the lowtemperatures being in the glassy state in midwinter.Some sort of potatoes can be hardened to withstandlow temperatures by gradual lowering their storagetemperatures. Certainly, it is possible to add otherexamples of glassy state occurrence in plant speciesin the nature; however, the knowledge of conditionsat which the glassy state is induced is not fullyunderstood though the glass formation in plants canbe of help to control the ‘cryogenic status’ at whichthe optimal undercooling achieves a suitable cryo-preservation of plant germplasm.

The process of vitrification [6] is, however,closely associated with the anomalous behavior ofliquid water [7], its apparent tendency to pentamericorganization within the liquid state at decreasingtemperatures as well as its possible interaction [8]with plant cells, which structures possess fractal andself-affinity architecture [1, 8, 9], cf. Fig. 1, thereforeirreconcilable to yield focal points for the icematching nucleation. Consequently, it seems to benecessary to reiterate some basic data about thestructure of liquid water, its structural quasi-periodicity, which becomes factually important forthe formation conditions of quasi-crystalline nucleiand their capability to form either state: biologicallyoptimal glass or hazardous intracellular ice.

Structuring in the undercooled water andits impact to the glass-formation tendency

It is clear that life on Earth depends on the unusualstructure and as all organisms consist mostly of liquidwater. This water performs many functions and it cannever be considered simply as a kind of inert diluentsor filler, because it carries the transports, makeslubrication, helps reactivity, evaporative cooling,stabilizes makeup, structures and partitions arrange-ment and may exhibit different character on thecontact with biological surfaces or within

membranes. The living world should be thought of asan equal partnership between the structure ofbiological molecules and the architecture of waterclusters. Recently the water was also shown to be thebest vehicle medium when assuming proton-issuedquantum diffusion [10]. In spite of numerous studies,many of the properties of water are yet puzzled notmentioning yet unidentified relations to livingorganisms [11]. Enlightenment comes from a betterunderstanding that water molecules form an infinitehydrogen-bonded network with confined and struc-tured clustering which become more intensive andbetter localized under decreasing temperatures.Recent studies done via molecular dynamics [12]identified experimentally water clusters, which growin size and become more compact as temperature de-creases. Their size can be characterized by a fractaldimension consistent with that of lattice animalshaving the correlation length proportional to theconfigurational entropy. Upon cooling, ice-likecrystals were more easily transformed from thesmaller clusters of (H2O)n with n up to 20, more likely

2 J. Therm. Anal. Cal., 88, 2007

�ESTÁK, ZÁME�N�K

Fig. 1 Some constructions (associable with the clusterstructure of liquid water): a – Dodecahedron, whichPlato associated with the firmament, b – the models ofcyclic pentamer and tricyclo-decamer, which are arelatively stable small clusters of water easyinterconnecting with the larger clusters of yet belowc – icosahedron (which geometry Plato associated withwater). d – Diamond (so called Penrose) basic tileswith a specific shape called rhombus can be expandedto cover greater areas, g – forming an unbrokenstructure where we can localized both the chains (likepolymeric water) and h – pentagons (like waterclusters, where the below connectivity map of thewater molecules shows the molecules orderlinesswithin an icosahedron). Such a structuring can also beapplied to a spatial distribution if the two kinds off – rhombohedra are assembled to form icosahedronmatching thus the larger clusters of water discussed inthe text in more details

Please,

shorten

the

caption!

existing at temperatures �5°C, while the largerclusters (n�20) were readily altered to glassystructures at temperatures ��0°C [13]. The apparentdecrease of the glass transition temperature with theincreasing cluster size was experimentally authorizedto be fewer effective than the corresponding changeof melting temperatures. Consequently, the mutualsuccession of the melting and glass-transitiontemperatures was found even inverted when com-pared with that observed for bulk water.

In order to get a better understanding ofprocesses of water crystallization and/or vitrificationit is necessary to look at the cluster sizedevelopment [14]. Small standard assembles of thefour-bonded molecules of liquid water may join atlowered temperatures together to form cyclicpentamer, tricyclo-decamer (cf. Fig. 1b) and largebicyclo-octamers, which arises from the competitionbetween maximizing Van-der-Waals interactions(higher orientation entropy and thus density) andmaximizing hydrogen bonding. Though cohesive innature the hydrogen bonding is curiously holding thewater molecules apart, which endows water withmany of its yet unusual properties. The tricyclo-decamer and by themselves forming a well symmetricbut spatially quasi-periodic [(H2O)280]x icosahedralmacromolecules may contain a variety of sub-structures being able to interlink and tessellatethroughout the volume. A mixture of watercontaining cyclic pentamers and tricyclo-decamerscan bring about similar ensuing clusters, whichprovide aperiodic tiling (i.e., a continuous pattern thatnever repeats exactly).

Some constructions, which are associable withthe cluster structure of liquid water can be foundtiling in a 5-fold symmetry that were thought, for longrun, as impossible to fill areas completely andregularly [8], Fig. 1. Upper thick rhomb (d) has longerdiagonal equal to the famous ‘golden ratio’ phi(�=1.618034..), which is related to the number 5 byformulae (1-�5)/2. Fascinatingly, it is playing acrucial role in various aspects of natural creations andart constructions. The thinner rhomb has his shorterdiagonal equal to 1/� and both rhombs can be derivedfrom a shaded area of pentagon (e), which all fivediagonals match � and which 5-side structure leavesgaps when used to be continuously repeated in space.On the other hand, the derived rhombs can fill thesurface in an asymmetrical and non-replicatingmanner (Fig. 1g), which is known as a continuous butnon-repeating structure [8] (sometimes called ‘quasi-crystals’ when recently discovered in quenchedglassy alloys). On expanded tiling to cover greaterareas, the ratio of the quantity of thick rhombs to thinones approaches � again and if the rhombs are

marked by shadow strips they form the unbrokenstructure (g) where we can localize both the chains(like polymeric water) and pentagons (like waterclusters), where the below connectivity map of thewater molecules shows the molecules orderlinesswithin an icosahedron (h). Such a structuring can alsobe applied to a spatial distribution if the two kinds ofrhombohedra are assembled (Fig. 1f) to form ico-sahedron matching thus the larger clusters of waterdiscussed in the text in more details.

All of three basic clusters are comparativelystable and it is expectable that their interaction canproduce larger embryos of icosahedra (IS), whichgeometry was already foretold as a water geometricalrepresentation by the Greek philosopher Plato(427-347 BC) [9, 15], Fig. 1c). Dynamically it formsa continuous but quasiperiodic network of open,low-density and condensed structures. Such afluctuating and rather self-replicating network ofwater molecules [16], with localized and overlappingof clusters, can interconvert between lower andhigher density forms by bending some of thehydrogen bonds but not by their interruption.

Less common water dodecahedra (cf. Fig. 1a)have been found in various aqueous solutions(curiously in the gas phase, too) and such clusterswere observed at hydrophobic and protein surfaces,where low-density water with stronger hydrogenbonds and lower entropy has been noticed.

Water clustering and biological systems

There exists a certain dynamical equilibrium with itscollapsed, but more frequent form of more symmet-rical dodecahedra (CS). The position of the IS�CSequilibrium around biological molecules wasrevealed to be important for their proper activity,e.g., with the enzyme balanced between flexibility(IS-environment) and rigidity (ES-environment).Character of water is critical not only for the correctfolding of proteins [17] but also for the maintenanceof this structure due to the impact of heat, i.e., de-naturation and loss of its biological activity, which islikely linked to the breakup of the 2-dimensionalspanning water network around the protein (due toincreasing hydrogen bond breakage with temp-erature) restrictive on the protein vibrationaldynamics. As the temperature increases the averagecluster size, their integrity and the proportionality inlow-density forms decrease and warmer water(�15°C) thus develops a predisposition to solidify toice-crystals more promptly upon quick cooling thancolder water does. Therefore the highly clusteredwater near freezing becomes incommensurable withhexagonal ice, which effect is the factual source for

J. Therm. Anal. Cal., 88, 2007 3

BIOLOGICAL GERMPLASM EXPOSED TO ULTRA-LOW TEMPERATURES

easy undercooling of the liquid water but, on the otherhand, does structurally correlate with the non-crystalline state of glass. This is not only importantfact for a quick preparation of ice in a refrigerator butexplains the higher sensitivity of plants to freezedown under big temperature drops when temperaturestarts to decline from higher temperatures.

The water clusters can tessellate in threedimensions as each cluster has twelve potential sitesat its icosahedral vertices to be exploitable as centersfor the neighboring overlapping clusters. However,when the network grows the structure, only locallysymmetrical, becomes more distorted in longer order,and expectably, the structure then becomes quasi-periodic while true tessellation can be possiblyachieved by the clusters pulsating between expandedand collapsed structures. Though such an icosahedralcluster is capable of continuous but non-periodictiling it is incapable of direct tessellation to such acrystalline structure that is needed for the congruousnucleation of hexagonally ordered ice due to itsresidual (and resistant) five-fold symmetry. Collapseof all four dodecahedral structures would be expectedto increase the density about a further three-fold formthat between the (ES) and (CS) structures to gives adensity of about 1.18 g cm–3 close to the 1.17 g cm–3

[18]. Such a collapse also gives an increase in theclosely approaching the so-called ‘interstitial’ water,when some hydrogen-bonds are removed from givenwater molecules. Therefore it would not be surprisingthat the neighboring organic molecules candrastically change the conditions for water makeover,particularly if scanning the fractal surface image oftheir biological interfaces, which tends to exhibit thepentagonal-like symmetry, too, yet supporting abovementioned incommensurability equally applicable toan interfacially induced heterogeneous nucleation ofice [19, 20]. The icosahedral structure contains largethat may allow occupancy by suitable solute(e.g., sucrose) or gases (CH4) and the vicinal water onsolid surfaces uses to exhibit further, rather curious‘extra-transitions’ to occur roughly at temperatures15, 30 and 45°C, possibly caused by the gradualbreakage of hydrogen bonds due to the differences insurface potentials.

Let us mention that all of the natural ice on theearth is hexagonal ice, which is well manifested in theexistence of six-cornered snowflakes. However, atlower temperatures and at higher pressures (above2 kbar) many other ice phases with different crystallinestructures exist, including various forms of glasses(phase diagram reveals it at about –130°C). No otherknown substance exhibits such a high variety of forms.If water vapor is forced to condense on a cold substratebelow –80°C, a cubic modification of ice is formed,

which is related to the classical ice in the same way as acubic diamond is related to a hexagonal diamondhaving also almost the same density. However, below–110°C a non-crystalline amorphous solid known aslow-density glassy ice appears.

For biological purposes there is importantelectrical conduction in ice, which is carried by theprotons, whose motion is closely tied to the motion ofionic defects. Since the small proton jumps occurcollectively along a favorably oriented chain ofH-bonds, the ionic defects and the effective chargecarried by them have a very high mobility in the icestructure, ten times greater than the mobility of ions innormal ionic conductors. Protonic conduction [10]plays a major role in biological charge transferprocesses across membranes or along nerves and theproton conduction in ice and in other H-bondedmaterials is periodical in nature [9] and analogous toelectron conduction in semiconductors. Besides theprotonic quantum behavior the self-organization cancommence with the soluble cations of sodium andcalcium. These cations exhibit most excellentdecoupling from the classical noise (of thermallyagitated mechanical impacts) so that they can movemore straightforward in aqueous solutions being thus‘quantum’ sensitive [1, 9, 10, 21].

Also the surface of ice shows unique properties asnear the melting point, the surface contains manydangling broken bonds that promote the existence of aliquid-like layer (needed for skating) also of impor-tance to biological contacts. In a whole class of solids,such as clathrates and all biological molecules, wherethe ice forms a H-bonded host lattice, that can encage agreat variety of small guest atoms or molecules likeargon or methane (large resources of methane hydratesdeposited at higher static pressure of water on the seabottom). Many forms of ice can be viewed asself-clathrates, where two equal ice lattices areinterpenetrating each other but are not H-bonded toeach other. Ice-like structures or structurally adjustedwater molecules are encountered in hydrates, andhydrated compounds including all macromoleculesthat would not be biologically active without thepresence of their structured water or ice-like shells,which can be viewed as a partially broken structure ofdistorted ice [22, 23]. Surely, there are many otherphenomena, which can strongly effect the character ofliquid water and its clustering behavior as that all isfalling, however, beyond the scope of this brief article.

Natural plant behavior while exposed tofreezing

Woody plants tolerating ice crystals in their tissueshave two major strategies to survive temperatures

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below zero. The first living strategy is aimed to avoidfreezing by either undercooling or lowering ofmelting point of their tissues and organs [24–26].Undercooling is a known effect in many plants andplant tissues, such as parenchymatic cells [27], wholeorgans as generative buds of [28, 29], or vegetativebuds of [30]. The level of supercooling goes down to–41°C [31] as the solid hexagonal ice have thestructure that is uncanonical with respect to the penta-gonal-like symmetry of a solidifying liquid water thatcontains a high number of incommensurable nuclei oficosahedra and dodecahedra.

The second living strategy of plants is toleratingof extracellular freezing [32, 33]. Survival of suchplant cells/tissues is based on their tolerance toexcessive dehydration of the protoplast. Some planttissues survive temperatures even below –40°C in thenature and in the techniques used for preservation ofplant genetic material they survive cryo-temperaturesas low as –196°C. The third strategy of plants tooutlast the sudden frost is the process called extra-organ freezing [32]. This type of strategy protects thewhole plant organ, such as bud, against ice nucleationand ice spreading and the external ice formationexposed the bud to frost dehydration [33]. The abovementioned survival strategies of plant species whiletolerating ice in their tissues can become combined,for example, the woody plants can exhibit extraorganfreezing in their buds, extracellular freezing in theirbark tissues and supercooling in their parenchymaticcells of xylem rays [34]. The question is what the stateof water or, better to say, the state of protoplasma is inthe plant cells subjected to subzero and lowtemperatures. Physical rules states that undercooledliquid can change into the form of glass during theprocess of glass transition at the narrow temperaturerange (Tg). The glassy state found in dormant twig ofa poplar tree [33] offers the explanation of what statesof solid water can be found in frozen tissuessupposing that this state occurred thanks to freezingdehydration of tissues.

The aim of our ongoing project is aimed to thecharacterization of the state of water in some selectedspecies specified for the germplasm storage in ultralow temperatures (liquid nitrogen) such as appledormant bud or garlic shoot tips [3].

Peculiarities of thermal analysis applied tocryogenic samples

Thermal analysis methods [1, 6] are of a favorite use inthe study of thermal behavior of both phenomenaobserved under the process of rapid cooling, ultra lowtemperature storage and re-heating. We are particularlycurious about the determination of concen-

tration-temperature dependences of glass transition aswell as the examination of phase relations in thesimulated systems such as aqueous carbohydratemixture [35, 36]. The shoot tips of garlic (Alliumsativum L. landrace Djambul 2) dissected from bulbletswere dehydrated over various saturated salt solutions ofknown water activity until the steady state of water wasreached. The temperature of glass transition wasmeasured in a heat flow type calorimeter, during coolingand warming with rate of 10°C min–1 rate. For highersurvival of garlic shoot tips after dehydration andcooling/warming we often used alginate bead withencapsulated shoot tip during dehydration. A typical

J. Therm. Anal. Cal., 88, 2007 5

BIOLOGICAL GERMPLASM EXPOSED TO ULTRA-LOW TEMPERATURES

Fig. 2 Thermal characteristics of shoot tips dissected frombulblets of Allium sativum L. landrace Djambul 2measured by DSC. The endothermic peak wasmeasurable at water content 0.323 gH2O/g dry masses.After dehydration to the lower water content(0.234 gH2O/g dry mass) the glass transition (Tg) withtypical difference in a heat capacity occurred. Themeasurement were made during heating 10°C min–1

(after previous cooling at 10°C min–1)

Fig. 3 Typical result of DSC study providing the plot of thedegree of dehydration vs. the glass transitiontemperatures (Tg’s). The shoot tips of garlic (Allium

sativum L. landrace Djambul 2) dissected from bulblets were dehydrated till the equilibrium with wateractivity of vapor above saturated salt solutions. Theglass transition temperature was measured, duringwarming

output from our DSC measurements is depicted inFig. 2. The development of temperature of the glasstransition, shows (Fig. 3, shows relationship with thewater activity of the plant shoot tips).

Conclusions

In the outlook of present vast progress in genomics, thedeterioration effect of changing environment and theongoing disappearance the biodiversity of plantspecies there is an important appeal for the exigency tostore plants with their complete genetic informationtypically inherent in the original wild-plant samples.The convenient cryogenic methods need, however,allocation of addition data of the thermal behavior ofspecies subjected to the process of thermal quenchingand consequent reheating. The measurement of glasstransition temperature by DSC/DTA is a good tool forestimation of optimal level dehydration to the optimallevel for cryopreservation. The information about glasstransition temperature also serves for checking ofcryopreservated plant material during storage in liquidnitrogen.

Portrayal of nonperiodic but continuous patternsof water on basis of variously shaped tiles [8] may beof help in other geometrical models used, e.g., in thephenomenological description of reaction mechanismin chemical kinetics [1, 9, 37].

Acknowledgements

This study was supported by the grant projectNo. 522/04/0384 of the Grant Agency of the Czech Republicand by the research project of FZU No. AV0210100521.The authors thank to Mr. Ivan Hon for redrawing the figures.

References

1 J. �esták, ‘Science of Heat and Thermophysical Studies:a generalized approach to thermal analysis’, Elsevier,Amsterdam 2005.

2 J. Záme�ník and A. Bilav�ík, Proceedings of the13th Congress of the Federation of Europe Societies ofPlant Physiology. Heraklion, Crete Greece 2002, p.773.

3 J. Záme�ník, M. Grospietsch and A. Bilav�ík,Cryobiology, 43 (2001) 328.

4 J. Záme�ník, A. Bilav�ík, M. Faltus and J. �esták,CryoLetters, 24 (2003) 412.

5 A. Bilav�ík, J. Záme�ník, M. Lázni�ka and J. �esták,Proceedings of: 8th European Symposium on ThermalAnalysis and Calorimetry (ESTAC), Barcelona, August2002, Abstracts p. 56.

6 Z. Chvoj, J. �esták and A. T�íska, Kinetic Phase Dia-grams: non-equilibrium phase transitions, Elsevier,Amsterdam 1991.

7 J. Buitink and O. Leprince, Cryobiology, 48 (2004) 215.

8 R. Penrose, The Emperor’s New Mind Oxford 1989;M. Gardner, Penrose Tiles to Trapdoor Ciphers, New York1989; H. O. Peitgen, H. Jurgen and D. Saupe, Chaos andFractals: new frontiers of science, Springer, New York 1992.

9 J. �esták, Heat, thermal analysis and society, NucleusPubl. House, Hradec Kralove 2004.

10 J. J. Mare�, J. Stávek and J. �esták, J. Chem. Phys.,121 (2004) 1499.

11 C. H. Cho, S. Singh and G. W. Robinson, FaradayDiscuss., 103 (1996) 19.

12 N. Giovambattista, S. V. Buldyrev, H. E. Stanley andF. W. Starr, Physical Review, 72 (2005) 11539.

13 A. V. Brodskaya and E. N. Laaksonen, Molecular Physics,100 (2002) 951.

14 D. Eisenberg and W. Kauzmann, The structure andproperties of water, Oxford University Press, London 1969.

15 J. �esták and R. C. Mackenzie, J. Therm. Anal. Cal.,64 (2001) 129.

16 M. F. Chaplin, Biophys. Chem., 83 (2000) 211.17 J. Swenson, H. Jansson and R. Bergman, Phys. Rev. Lett.,

96 (2006) 240.18 O. Mishima, L. D. Calvert and E. Whalley, Nature,

314 (1985) 76.19 P. G. Kusalik and I. M. Svishchev, Science, 265 (1994) 1219.20 S. Mashimo, J. Non-Cryst. Solids, 172/174 (1994) 1117.21 J. J. Mare� and J. �esták, J.Therm. Anal. Cal., 82 (2005) 681.22 H. Suga, Thermochim. Acta, 300 (1997) 117.23 L. S. Bartell, J. Phys. Chem., 101 (1997) 7573.24 E. N. Ashworth and M. E. Wisniewski, Proceedings

‘Breeding fruit crops for cold climates’, 85th ASHSAnnual Meeting/33rd CSHS Annual Meeting East Lansing,Mich. 1988. HortSci, 26 (1991) 501.

25 M. J. Burke, L. V. Gusta, H. A. Quamme and C. J. Weiser,Ann. Rev. Plant Physiol., 27 (1976) 507.

26 S. R. Malone and E. N. Ashworth, Plant Physiol.,95 (1991) 871.

27 Z. Ristic and E. N. Ashworth, Plant Physiol., 104 (1994) 737.28 M. R. Warmund, M. F. George and B. G. Cumbie,

J. Amer. Soc. Hort. Sci., 113 (1988) 418.29 A. Bilav�ík and J. Záme�ník, Biologia, 51 (1996) 62.30 A. Sakai, Cell Physiol., 23 (1982) 1219.31 A. Sakai, Plant Cell Physiol., 20 (1979) 1381.32 L. Chalker-Scott, Annals of Botany, 70 (1992) 409.33 A. G. Hirsh, R. J. Williams and H. T. Meryman, Populus.

Plant Physiol., 79 (1985) 41, Cryobiology, 24 (1987) 241.34 M. Ishikawa and A. Sakai, ‘Characteristics of freezing

avoidance in comparison with freezing tolerance: a dem-onstration of extraorgan freezing’ In P. H. Li and A. Sakai,Eds, Plant Cold Hardiness and Freezing Stress.Mechanisms and Crop Implications, Academic Press,New York 1982, p. 325.

35 J. F. K. Schawe, Proceedings of NATAS’06, BowlingGreen, USA 2006.

36 A. Sikora, V. O. Dupanov, J. Kratochvíl and J. Záme�ník,J. Macromol. Sci., B: Physics, 46 (2007) 71.

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DOI: 10.1007/s10973-006-8232-8

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9. PROPERTIES OF SOME NATURAL GLASSES: AUSTRALIAN OPALS AND CZECH TEKTITE MOLDAVITES

Paul Thomas, Klaus Heide, Jaroslav Šesták, Ekkehard Füglein, Peter Šimon

9.1. Origin of natural inorganic non-crystalline solids

Vitreous and amorphous silicates are commonly found on the Earth and lunar surfaces in a variety of geological environments. As the vitreous (or amorphous) state is a state which contains no structural order, classification of non-crystalline solids is most easily and commonly made by relative comparison of the structural framework at the atomic scale and, as shown by Trömel [1] in terms of short and long range order which results in four generalised states schematically shown in Fig. 9.1.

The top left corner has the lowest degree of order in the solid state and is defined as the amorphous state where there is no long or short range order and represents a dynamic condensed liquid-like phase where relatively long scale segmental motion exists. Ductile solids and gel like solids may be placed in this category. The bottom right of the diagram represents the crystalline state which, of course, contains a high degree of structural order both in the short and long range. In between these extremes are degrees of structural order which form at the short range and long range and are the topic of discussion of this chapter.

The glassy state is, in a sense, a specific subset of the non-crystalline state and contains no long range order, but its short range order is increased by the freezing of gross molecular or segmental motion. The freezing of molecular motion results in a characteristic brittleness of the glassy state. A subset of the glassy state is the vitreous state which encompasses materials that are cooled rapidly from the melt at a rate that inhibits crystallisation. Tektites, such as moldavite, are examples of such materials produced naturally

Fig. 9.1. Structural classification of non-crystalline and crystalline solids in terms of long and short range order (taken from Ref. [2]).

Fig. 9.2. Schematic T-t-plot of glass-forming processes in nature and industry (taken from Ref [2]).

through fusion of rocks and rapid quenching of melts during the high energy impact of meteorites with the Earth’s surface. Magmatic and metamorphic processes are also important sources of the vitreous state.

The high temperature processes responsible for the formation of the vitreous state are not the only processes which result in amorphous, glassy solids. Low temperature diagenetic and biotic processes also result in amorphous, glassy solids; hyalite and ‘potch’ opal (opal that is not opaline) are examples. In rare cases, the diagenetic process results in the opaline state. The opaline state is a state containing a low degree of short range order while attaining a high degree of order at long range. The opaline state is so called because of the ability of the ordered long range microstructure to diffract visible light giving the observer the impression of dancing colours or play-of-colour (POC) resulting in opalescence. In principle the second order structure is not influenced by the primary structure at short range and, hence, the short range order at the atomic level may range from the amorphous gel like structures of sedimentary opal to the paracrystalline structures observed in volcanic opal.

Natural inorganic glasses, such as the vitreous glasses or the opaline glasses produced by diagenetic processes may be found widely distributed across the Earth’s surface. The quantities of these naturally produced glassy solids is dependent on the formation process and ranges from micrograms up to kilo tonnes and, hence, the occurrence is from microscopic glassy inclusions to “glassy mountains” [3]. Much of the natural glasses are vitreous materials which have been quenched from the melt. The thermal history of glass forming processes in nature varies significantly from the conditions used in the glass industry which are optimised between processing speed and energy conservation (Fig. 2). In the extremes, tektites like moldavites are formed by extremely fast heating and melting at very high temperatures (> 3000 K) with consequent extreme cooling rates (≥ 106 K/sec). By contrast the formation of amorphous glasses from mineral diagenesis or biotic processes occurs at ambient temperatures; the formation processes are essentially isothermal and take place over long periods of time of the order of months to years or at extremely low cooling rates (K/year) (Fig. 9.2).

In contrast to industrially produced glass, the chemical composition of the majority of natural vitreous glasses is silica rich (> 70 % SiO2), peraluminous (i.e. Al2O3 > Na2O + K2O + CaO) and is principally characterized by a rhyolitic composition. The low diagenetic or biotic glasses tend to have even higher silica contents which can approach 99% of the anhydrous

composition and are also peraluminous. The high silica and alumina content of natural glasses results in chemical and physical properties which differ significantly from industrial glass, including glass transformation, nucleation and crystallization, the H2O content and long time durability. For example, water contents range from 0.01% in tektites to between 3 and 13% (or greater) in opaline “glasses” which compares with approximately 0.2% in industrial glass.

The vitreous or amorphous state is inherently frozen-in thermodynamically as there is a tendency towards crystallisation or devitrification as well as the potential for weathering. Industrial glasses are susceptible to weathering and degradation at relatively short periods of time (<1000 years). Specimens of natural vitreous or amorphous materials, however, are known to have significant durability. For example, in relatively inert environments, such as on the surface of the moon, vitreous solids have existed for more than a billion years. This inherent stability may be ascribed to the high silica and low alkali content of natural glasses.

Given the extremes of formation, durability and property of natural glasses, this chapter focuses on two types of natural vitreous or amorphous solid states which correspond to two of the four structural orders discussed above; Australian opal which has the opaline structure and the tektite moldavite which is a vitreous glassy material. Both of these materials are rare non-crystalline materials and are prized primarily for their aesthetic value and for use in jewellery. 9.2. Australian Opal

The history of opal is a little confused by the current and historical meaning of the word “opalus” or its modern Latin derivative opal. Some texts suggest that opal as a gemstone has been mined and valued since Roman times. It is however likely that Roman opal refers to a number of glassy minerals showing brightness [4]. The first recorded systematic mining of opal (in its current meaning) began in the 17 century in north-eastern part of Hungarian Empire which is now Eastern Slovakia. Precious opal, that has the opaline structure and displays opalescence or play-of-colour, became an important and popular gem of the Royals of Europe during the 18th and 19th Centuries when the Slovakian mines were at their most productive. During the 19th and 20th Centuries commercial opal deposits were found in Australia, which has since dominated world opal production, Mexico, Peru, Brazil, Honduras and Indonesia amongst other localities. 9.2.1. Basic geology of Australian opal

In Australia, the commercial production of precious opal occurs mainly in the sedimentary environments and accounts for as much as 90% of total world precious opal production [5]. The distinction of the sedimentary environment is important as the sedimentary nature is a predominant characteristic of the commercial Australian opal fields. Most of the production from outside Australia is derived from volcanic environments and, hence, volcanic host rocks such as andesite and rhyolitic tuff [6-8].

The Australian sedimentary geological environment is associated with the Great Australian (Artesian) Basin (GAB); the opal forms within the weathered sedimentary profiles that make up the GAB. In Coober Pedy, South Australia, for example, the opal-bearing stratum consists mostly of clay stones that form the Stuart Range, an east facing escarpment along the western margin of the GAB. Opal is found associated with the chemically weathered rocks that comprise the Bulldog Shale which is part of the Marree formation and is of early Cretaceous age. The Bulldog Shale is subdivided into two stratigraphic units; an intensely weathered and bleached claystone (called sandstone by the miners) and an unweathered to partly weathered claystone. Precious opal is most often found at the interface between these two stratigraphic units. Whilst this is a simplified description of geology of opal deposits, a correlation between these layered weathered profiles and the profiles in the

(a)

other South Australian fields as well as in the Lightning Ridge (NSW) and White Cliffs (NSW) opal fields is evident [9].

Similar features are found in the main opal producing volcanic environment in Australia in the Tintenbar district of north eastern NSW. The occurrence of the Tintenbar ‘volcanic’ opal represents, commercially, the largest producing district of volcanic environment precious opal in Australia, although its production is small when compared to the sedimentary environments. In Tintenbar, the opal is associated with the decomposed basalt of the Lismore Basalt and part of the Lamington Volcanics of Miocene age. In this area, the Lismore volcanics consist of three distinct volcanic flows. The precious opal occurs

(b) (c)

Fig. 9.3. (a) Image of the mullock heaps (mine tailings) around the 3 foot holes in Coober Pedy (left), and examples of rough cut opal specimens from Andamooka, South Australia (top right) and Koroit, Queensland (bottom right). SEM micrographs of hydrofluoric acid vapour etched fracture surfaces of (b) a Coober Pedy white play-of-colour (POC) opal and (c) a Tintenbar crystal opal (taken from Ref [14]).

mainly near the junction of the first and second basalt flows as loose nodules in the soil and as amygdales in the decomposed vesicular basalt [10-11]. 9.2.2. Origins of the silica of precious Opal

Precious or noble opal is prized for its play-of-colour, the origins of which are based on the diffraction of visible light off ordered arrays of monodispersed silica spheres [12-13]. Precious opal is an unusual material as it is composed of amorphous hydrous silica with the general formula SiO2.nH2O, but is precipitated as monodispersed colloidal particles from 150 to 400 nm in diameter. The monodispersed colloid is then concentrated by sedimentation, evaporation or by filtration under pressure to form the ordered arrays of the monodispersed silica spheres that are responsible for the diffraction of light observed (Fig. 9.3b).

The monodispersed silica colloid is formed through the dissolution and precipitation of silica. A likely source of the silica is from the weathering of silicates such as feldspars [13, 15]. In the weathering model, the source of the silica is the sandstones associated with the GAB where the chemical weathering of relatively soluble silicates such as the feldspars contained in these sediments results in the formation of an alkaline silica solution. An example of such a mineral is potassium feldspar which weathers through the idealised stoichiometry: 2KAlSi3O8 + 3H2O → Al2Si2O5(OH)4 + 2KOH + 4SiO2 (9.1) by the permeation of ground water through the sediments resulting in kaolinite, dissolved silica and an increase in pH through the release of potassium hydroxide. Reported trace element distributions in opal from a wide variety of sources are consistent with such a weathering model [16-18]. Once the silica is in solution, the enrichment of the solution by evaporation or filtration can occur. Increasing the concentration of the silica solution coupled with a lowering of the pH through alkali ion exchange with the surrounding clays allows the nucleation of primary silica spheres and, subsequent, sphere growth as more silica is supplied to the system. The supply of silica in the solution also appears to be a cyclic process as generations of growth rings are observed in the silica spheres (Fig. 9.3(b)). Once the monodispersed silica colloid has reached a suitable size, concentration of the silica colloid is then required and in certain cases, ‘crystallisation’ of the monodispersed colloid occurs to form the ordered arrays which results in the prized play-of-colour. The interstices are subsequently in-filled with a silica cement completing the formation of the opal producing a hard material which, despite the gel-like structure, has a Berkovich hardness of 5.7 to 6.2 GPa which is similar to that of soda glass (6.4 GPa), but, as might be expected for a gel like material, is significantly less than that of fused silica (10.8 GPa) [19].

Although the weathering model is important in aiding the understanding of the formation of opal, a number of other models have been proposed to account for specific observations in specimens acquired by the authors of these models. Examples of such models are the “microbial” model where microbes have been observed in Lightning Ridge matrix specimens suggesting that microbial action is responsible for the source of the silica [20], the syntechtonic fluid model where high pressure, warm hydraulic silica rich fluids are the silica source [21] and the mound spring model where the silica rich alkaline waters derived from the artesian basin well up through the mound springs supplying the silica rich solutions required for opal formation [22]. Although these models source the silica from different origins, the formation of the monodispersed colloid must occur by homogenous precipitation followed by a concentration mechanism such as evaporation, filtration or sedimentation.

A sedimentation mechanism for the formation of opal does have some support [18, 23]. Elemental distributions in banded opal have suggested that although the bands must be

Fig. 9.4. X-ray diffraction patterns for (a) a Coober Pedy white play-of-colour (POC) opal and (b) a Tintenbar crystal opal. The Tintenbar opal show peaks characteristic of opal-CT while the amorphous ‘hump’ of the Coober Pedy opal is characteristic of opal-AG [28].

formed from the same solution (e.g. constant aluminium concentrations across bands), the variation in ion concentration between bands of highly charged metal ions suggests that the bands form by flocculation of the silica colloid. Leisegang type phase separation (repeating colour rings) on precipitation of the silica may also explain the concentration variations of trace elements between bands.

The ‘sol-gel’ nature of the formation of opal is responsible for the hydrous gel-like structure of the silica and as a consequence opal contains a significant proportion of water much of which is reported to be molecular [24-25]. Four types of water have been postulated; molecular water surface adsorbed (e.g. pore), bulk cage molecular water, surface silanol water and bulk silanol water [15]. Quantification of the amount of each species has proven to be difficult; however, Langer and Flörke [25] from their infrared spectroscopic studies suggested that approximately 90% of the water contained in sedimentary opal was present as molecular water which confirmed Segnet et al’s [24] postulation based on the calculation of surface area of the spheres that comprised play-of-colour opal. Further confirmation has been supplied by 29Si NMR measurements where approximately 10% and 26% of the water was estimated to be in the form of silanol groups in Coober Pedy white POC opal and Tintenbar crystal opal, respectively [26-27]. Thermal analysis has also demonstrated the presence of a significant proportion of molecular water in both volcanic and sedimentary opal [28]. 9.2.3. Morphology of Opal

The environment in which opal is formed is important as can be discerned from the differences in physical properties of opal derived from the volcanic and sedimentary environments and, in particular, the morphology of the opal [7, 14, 28]. In general, hydrous opaline silica has been found to occur with a range of morphologies which, based on x-ray diffraction (XRD) measurements [29], were originally divided into three categories; crystalline opal predominantly composed of cristobalite, opal-C, partially crystalline opal containing XRD characteristics of cristobalite and tridymite, opal-CT, and amorphous opal, opal-A. Subsequent analysis by Langer and Flörke [25] resulted in a sub-division of opal-A into two further categories, amorphous network silica, opal-AN (e.g. hyalite), and an amorphous gel silica, opal-AG (e.g. precious opal displaying play-of-colour), based on their relative proportions of water and appearance. These forms of silica are related by their relative

Table 1. Physical properties of sedimentary opal-AG and volcanic opal-CT. Sedimentary Opal Volcanic Opal Refs. X-ray Diffraction (XRD) Opal-AG Opal-CT [25, 29]

Crystallinity Amorphous Partially crystalline [25, 29] Density ~ 2.15 ~ 2.00 [36]

solubility in water and their thermodynamic stability resulting in an Ostwald like succession for formation: Opal-AG → Opal-CT → Opal-C → microcrystalline quartz (9.2) which occurs under low temperature solution diagenesis [15, 30-33]. Australian sedimentary opal, opal-AG, may be considered as the first generation of silica formed by these low temperature aqueous processes as it is the first of the ‘pure’ silica phases that are produced from the weathering of basic aluminosilicates. Both sedimentary and volcanic opals typically contain approximately 1% trace element impurities based on the mass of silica present [16,18].

Opal derived from volcanic environments is generally found to have the opal-CT morphology. This significant difference in morphology demonstrates the significance of the environment on the formation of opal as can be seen from the typical XRD patterns shown in Fig. 9.4 [14, 28]. Although volcanic opal may be categorised as opal-CT based on the XRD data, the designation of volcanic opal as a second generation silica polymorph is contentious. As volcanic opal displays play of colour, it contains a monodispersed ordered array of silica spheres (Fig. 9.3c). This fact suggests that a monodispersed colloid is the precursor of the opal which suggests homogeneous precipitation and, hence, a first generation silica [14, 28]. Higher generation opal and silica polymorphs are formed as the equilibrium concentration of silica reduces; the solubility of each phase decreases according to the order represented by Eq. (9.2). The first generation of silica in Eq. (9.2) should, therefore, be extended to include precious play-of-colour volcanic opal-CT in addition to opal-AG as being formed by homogeneous precipitation with later generations formed through a heterogeneous nucleation and growth mechanism.

9.2.4. Physical properties of sedimentary and volcanic Opal

A number of observations and features distinguishing opals from the sedimentary and volcanic environments have been reported and are listed in Table 9.1. The water content of volcanic opals tends to be in the range 10 to 20% by mass while for Australian sedimentary opal the range is lower at circa 6 to 10% [12, 24-25, 34]. In keeping with the higher water content of volcanic opals, the refractive index is also lower for the volcanic opals [35]. Another interesting difference is the relative density [36]. Although the volcanic opals have a

NMR Less Q2, Q3 More Q2, Q3 [26-27] Water content 4% – 9% 9%-18% [25, 35] Water Types - Silanol More silanol - OH Less Silanol - OH [25-27] Water Types - Molecular Less Molecular – H2O More Molecular – H2O [25-27]

Refractive Index 1.42 -1.45 1.40 – 1.42 [35-36] Strong emission @ 2.65eV, excitation @ 5.0eV Photoluminescence Negligible [36]

(a) (b) Fig. 9.5. Thermogravimetric analysis data for Coober Pedy (sedimentary origin) and Tintenbar (volcanic origin) opals; (a) mass loss data, (b) differential mass loss (DTG) data, heated to 1000oC at 1K/min in an air atmosphere (taken from Ref [28]).

higher degree of order (i.e. opal-CT), the density of volcanic opal is usually around 2.00 g/cm3 while for the amorphous sedimentary opals the density is higher at 2.15 g/cm3. It is also interesting to note that sedimentary opals tend to be fluorescent while volcanic opals are not [36]. A further significant difference is in the manner in which the opals etch. Hydrofluoric acid (HF) vapour etched fracture surfaces of a Coober Pedy and a Tintenbar opal are shown in Fig. 9.3. Both opals show the distinct ordered array with the significant difference that it is the silica cement that is etched in the sedimentary opal while it is the spheres that are etched in the volcanic opal, a feature that has also been reported for Ethiopian and Mexican volcanic opal [37].

Thermogravimetric analysis has also shown significant differences in the character of the structure of sedimentary and volcanic opals. Coober Pedy white POC opal and Tintenbar crystal opal have been investigated using thermogravimetric analysis (TG). Mass loss data for specimens of each opal are shown in Fig. 9.5a for specimens that have been denoted as lumps and powder. Significant differences are observed both between the opal types and between lump and powder forms. In order to clarify the presentation of the data, differential TG (DTG) curves are shown in Fig. 9.5b. The peak position represents the temperature at which the maximum rate of mass loss, which for the opal is the maximum rate of water loss. For the Coober Pedy specimens, the peak position (measured as the centre of mass) for the lump specimens is at 267oC while for the powder specimens the peak position is at 190oC. This significant shift in the peak position is accounted for by the change in the geometry of the specimens. As the water in opal is predominantly present as molecular water, the shift in the peak to lower temperatures for the powdered specimen may be accounted for by the reduced path length required for the diffusion of the water molecules [38-39].

Further TG and differential scanning calorimetric (DSC) studies have been carried out on the CP white play of colour opal by heating to 1640oC (Fig. 9.6). The measurements were carried out at a heating rate of 20 K/min. The endotherm in the 1st heating run at 303oC corresponds to the maximum water loss and is observed at a higher temperature than in Fig. 9.5 due to the high heating rate (20 K/min as opposed to 1 K/min in Fig. 9.5). The conversion of opal to cristobalite during the heat treatment is observable in the 2nd heating run DSC curve, where a sharp endothermic peak is observed at 217oC corresponding to the α−β cristobalite transition. It is likely that this crystallisation is occurring from 1166oC where exothermic behaviour is observed in the 1st heating run DSC curve. This relatively easy crystallisation along with the amorphous peak centred on the cristobalite peak at 22o 2θ

0 200 400 600 800 1000 1200 14000,0

5,0x10-9

1,0x10-8

1,5x10-8

2,0x10-8

200°C

1219°C1024 °C

689°C

456°C

Opal Coober PedyAustralien3 mg

Water release

Ion

curr

ent m

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Fig. 9.6 (left) TG and DSC data of the CP white play-of colour opal heated to 1640oC at 20K/min in an air atmosphere. The 2nd heating run shows the � � cristobalite transition at 217oC suggesting crystallisation has occurred corresponding to the exotherm observed above 1166oC in the 1st heating run. Fig. 9.7 (right) Water release from Coober Pedy white play-of-colour opal under high vacuum between 20°C up to 1000°C at a heating rate of 10 K/min. The water loss corresponds to 99.63 wt% of the total mass loss. The spiky release of water between 456°C and 689°C is 0.29 wt% and between 1024 and 1219 °C is 0.08 wt% and corresponds to sudden releases from enclosed pores or fluid inclusions.

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(a) (b) Fig. 9.8. Hydrogen release with a maximum of 877°C and methane release with a maximum at 100oC for the Coober Pedy opal studied in Fig. 9.7.

suggests that the opal-AG (amorphous sedimentary opal) is tending toward the cristobalite structure. It is also indicative of the high silica content and low alumina content of opal (circa 99% and 1%, respectively, of the anhydrous mass). This propensity, however, does give credence to the models that suggest that precious opal-CT of volcanic origin is a second generation opal and conforms to the Ostwald ripening scheme of Eq. (9.2).

Although the water loss in Fig. 9.5 and 9.6 for the Coober Pedy white play-of-colour opal is apparently through a single peak, the desorption is actually more complex as shown by the high vacuum hot extraction DEGAS data displayed in Fig. 9.7 which was carried out at a heating rate of 10 K/min to 1400OC. The main peak of desorption is observed around 200oC which concurs with the TG data. The DEGAS data, however, also shows shoulder peak between 400 and 700oC and continued mass loss up to and beyond 1000oC which also corresponds to water loss although trace concentrations of hydrogen and methane were

Fig. 9.9. Images of typical Moldavites specimens conforming to the aerodynamic shapes expected from the quenching of the molten glass at high velocities.

detected (Fig. 9.8) amounting to less than 0.4% of the total mass loss. The evidence of higher temperature water loss suggests that the structure of the sedimentary opal is not as homogeneous as previously implied. It is also notable that the spikes in the data in Fig. 9.7 correspond to sudden losses which may be ascribed to sudden release from fluid inclusions or entrapped micropores in which the pressure escalated beyond the breaking stress of the opal. The hydrogen released around 880oC is likely to be derived from micellar decomposition of silanols [41], but the methane evolved around 100oC is a little more difficult to explain as the temperatures are too low for the micellar reactions inferred for the evolution of hydrogen. It is possible that very small proportions of molecular methane are trapped in the gel structure as the opal network is formed.

The characterisation of the materials properties of sedimentary opal and the differences in the properties between sedimentary and volcanic opal are important as they aid the further understanding of the processes involved in the formation of the opal. In general terms, the volcanic opal contains more water and is of lower density than the sedimentary opal. This suggests that the volcanic opal has more open structure. The HF etching (which will remove material that is more susceptible and, hence, less dense) removes the sphere silica in the volcanic opal also indicates a less dense silica network. These differences in properties may be associated with the temperature of formation, the concentration of electrolytes or the pH of the precipitating solution. The origins of the difference in property are not currently clear. However, given the differences in the physical properties observed between volcanic and sedimentary opals, the environment must play a clear and important role in the formation of opal. 9.3. Tektites – Moldavites

‘Moldavites’ (in Czech terminology “vltavín”) are specific forms of tektite which are prized for their unique shape, clarity and distinctive greenish colour resulting in a fascinating and beautiful gemstone [42-43]. It is one of the rarest “minerals” on the Earth, and was first found in a few limited areas in the south of the Czech Republic. Moldavites were described as early as in 1787 when they were listed among precious stones (used in jewellery which continues to the modern day) and are classed, by believers, as among the most powerful gemstone tools for spiritual expansion of consciousness and self-discovery (often associated with the legendary “Stone of the Holy Grail” - fallen from the sky and being a talisman for healing the Earth [44].

Tektites are centimetre to decimetre-sized bottle green to blackish glassy “bodies”. Tektite is derived from the Greek, tektos meaning molten and tekein to melt. They are found in gravels ranging in age from upper tertiary to alluvial in the Radomilice area, Chlum near

Fig. 9.10. The Moldavite strewn field showing the asteroid impact sites and regions where moldavites of consistent chemical composition are found (taken from Ref. [49]).

the Moldau river in Moravia, Bohemia, Czech Republic (Moldavites), in Lusatia, Saxonia Germany, in Indochina, Vietnam, Thailand, Malayan Peninsula, Java, Flores, Billiton, Borneo, Philippines, Australia and Tasmania (Australites), in Zhamnanshin (Irghizites) Kazakhstan, Ouellée on the Ivory Coast in Africa, Texas and Georgia in the United States (Bediasites). Microtektites (vitreous/crystalline spherules < 500μm) have been found in deep-sea deposits in the Gulf of Mexico, Caribbean Sea, north-western Atlantic Ocean and on Barbados. The chemical composition of microtekties is closely associated with the North American tektites. Transparent colourless to pale brown microtektites are also found associated with clinopyroxene spherules in the western Pacific and Indian Ocean in the upper Eocene marine deposits [45].

The origin of tektites was intensively studied and discussed in the last century, particularly in the latter decades, with respect to the mass extinction during the short time event known as the Cretaceous – Tertiary boundary. The discussion was dominated by two alternative origins of tektite material: the Terrestrial Impact Theory (TIT) and the Lunar Volcanic Theory (LVT) [46-47]). As Izokh [46] noted the TIT “won a complete victory over O’Keefe’s [47] Lunar Volcanic Theory” and, currently, the majority of scientific community discusses the origin of tektites and mass extinction event on the basis of TIT as, in particular, the chemical evidence supports the formation of tektites from terrestrial rocks and soils [48].

The terrestrial impact theory is based on the significant transfer of energy from impacting asteroids with the earth’s surface. As asteroids are massive, impact with the earth surface produces high energy collisions and high pressures and temperatures in the impact zone which results in the fusion and vaporisation of the material around the impact zone. The energy of these impacts was such that fused and vaporised material was catapulted out to near space. As this material returned to earth, it rapidly cooled and solidified with an absence of gaseous inclusions (although vacuous inclusions are observed). During the quenching process the solidifying phase was generally shaped aerodynamically and as the molten droplets were spinning, a range of bizarre shapes; plate, spheroid, rod, dumbbell, teardrop, star, curly, winkled and bubble, were produced (Fig. 9.9). As these particles were returned to earth, the particles were distributed by aerodynamic transport over large areas on the Earth-surface in strewn fields. The tektites found in particular strewn fields are postulated to have been derived

Table 9.2. Typical chemical compositions of Moldavites

Oxide Moldavites [49]

Australites Microtektites Obsidians Utah, [52] Australasian Yellow Stone Park,

Island n= 69 n = 32 [45] [53 p. 183]

SiO2 79.0 73.1 72.0 75.0 TiO2 0.3 0.7 0.8 0.11 Al2O3 10.3 12.2 13.6 12.5 Fe2O3 - 0.6 - - FeO 1.7 4.1 4.9 1.0 MgO 2.1 2.0 2.4 0.03 CaO 2.5 3.4 3.6 0.6 Na2O 0.4 1.3 0.6 2.9 K2O 3.4 2.2 1.9 5.1 P2O5 0.06* - - 0.1 H2O < 0.02+ - - 0.3 *[54], +[55]

Table 9.3 Element ratios of tektite and impactite and for several terrestrial and lunar materials. Material K/Sc K/Rb K/Zr K/Cs K/Ba K/U Ti/Zr Cr/Ni Rb/Zr Ba/Rb Ba/Zr

Moldavite 7.180 271 145 2.010 49 12.550 8.60 1.50 0.54 5.86 3.01 T8201* 6.400 240 135 2.060 45 12.200 8,60 1.40 0.56 5.64 3.10 Average

Earth 6.000 247 240 10.500 50 14.000 6.85 0.91 0.97 4.94 4.8 Average

granite Earth

10.700 179 49 - 358 23.800 6.81 17.5 0.27 O.50 0.14 Average sandstone Moon 4.37 296 5,93 6.920 9.43 2.515 129 0.020 31.4 0.63 bulk Apollo 15

- Green glass 345 7.33 - 11.1 0.0264 31.0 0.82

Apollo 12 - 327 3.88 - 6.7 0.0086 0.58 fines

* [51]

from the same impact event due to the high degree of compositional homogeneity within an individual strewn field (Fig. 9.10. [49]).

The impact-formed or metamorphic glasses have a significant durability. The age of impact-formed glasses at the earth’s surface has been determined to be between 700000 years (australites) and 34 million years (bediasites). In a more inert environment such as at the lunar surface, durability of the metamorphic glasses has been demonstrated by glass spherules which have been aged at more than 2 billion years. In general, recovered specimens of tektite have been observed to be resistant to hydration or devitrification of the bulk [50]. Only a characteristic corrosion of the surface is observed at the Bohemian tektites - the moldavites [42]. 9.3.1. The chemical composition of Tektites

The content of main elements of tektites is similar to the terrestrial volcanic glasses, in particularly to the obsidians. In terms of the geochemical nomenclature the “tektites” are

Fig. 9.11. (left) Thermal expansion of (a) obsidian, (b) Bohemian moldavite first heating, (c) Bohemian moldavite second heating, (d) Mahren moldavite and (e) fused silica (heating rate: 5 K/min) (taken from Ref [56]).

Fig. 9.12. (right) Density changes as a function of the cooling rates for known thermal histories are plotted to allow estimation of the cooling rates of the natural glasses during formation (taken from Ref [57, 58]).

peraluminous (Al2O3> Σ(Na2O + K2O + CaO) rhyolitic glasses (SiO2 > 70 wt%). The composition of a selection of tektites is given in Table 1. Although tektites have a similar elemental composition to volcanic rhyolitic glasses such as obsidians, a remarkable difference in the water content exists between tektites and obsidians (tektites < 0.01 wt%, obsidians > 0.1 wt%) and in the redox state ( in tektites Fe2+ >>> Fe3+, in obsidians Fe2+ ≥ Fe3+).

For the evaluation of the origin of natural materials, the trace element ratios are important tools. Correlation coefficients of geochemical well known pairs show a clear distinction between different bodies of the solar system. By these ratios (e.g. K/Sc; Ba/Zr, La/Yb), the tektites are chemical indistinguishable from terrestrial materials providing evidence that tektites originate from terrestrial materials and are likely to be of sedimentary origin (Tables 2 and 3 ) [50].

9.3.2. Physical properties of Tektites

The thermal properties of tektites are most easily characterised by the thermal expansion. Fig. 9.11 shows the thermal expansion of a series of natural glasses showing that the thermal expansion coefficient α for the moldavite (3.7 x 10-6K-1) is significantly greater than fused silica (0.5 x 10-6K-1), but less than that of obsidian (6.3 x 10-6K-1) and soda lime glass (7.5 x 10-6K-1) [2, 50]. The comparison with obsidian is most apt as both minerals have a similar rhyolitic composition (Table 9.2). The difference in the expansion coefficient is most likely due to the relative proportions of sodium and potassium with respect to calcium and magnesium within the silica network. The higher CaO and MgO and lower Na2O content of moldavites results in a more tightly bound structure resulting in a lower expansion coefficient. The glass transition temperature (Tg ~ 780°C) is also observed to follow this trend and is approximately 100K higher for moldavite than obsidian.

Although expansion coefficients of the moldavites are lower than those of obsidian and commercial glass due to the high alumina, calcium and magnesia content, the fictive temperatures of moldavites tend to be high and are higher than common industrial silica-glasses. The origins of the high fictive temperatures lies in the initial melt temperature and the rate of quenching. The high melting temperature produces a lower density melt which is rapidly cooled and, hence, the low density or high fictive temperature is preserved by the

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Fig. 9.13. (left) TG and DSC data for a moldavite specimen heated at 20K/min in and air atmosphere up to 1640oC showing little variation in the mass Fig. 9.14. (right) Plot of water evolution from a moldavite specimen showing traces of strong bounded OH-species released from the sample above 1200°C as water.

thermal history and relaxation of the lower density structure at ~ 600°C occurs as the moldavite is heated (Fig. 9.11).

These origins of the differences in fictive temperature can be confirmed by estimation of the cooling rates in the formation of the moldavites. A method for the estimation of cooling rates during the quenching of glass, known as the geospeedometric double-density technique, was developed by Klöss [58]. The method relies on the difference in density between the quenched and thermally annealed specimens of glass which produces a linear relationship that can be used to extrapolate to an unknown cooling rate based on a measured density. Fig. 9.12 plots the relative density of tektite and obsidian glasses as a function of cooling rate (i.e. model thermal history). By the regression of data in Fig. 9.12 the natural cooling rate q0 [K/min] may be determined from the experimental cooling rate q [K/min], a geospeedometric coefficient Θ and the determined density change ΔD/D, q = qoe-Θ·(δD/D) . In case of a magmatic glass like obsidian the natural cooling rate was determined ~ < 50K/a and for the tektite glass > 10K/s (Fig. 9.12).

Further evidence of the high cooling rates from high temperatures may be gleaned from the characteristic intermediate range order which, in glasses, may be defined by the magnitudes of two parameters a and f [56]. The “a-value” is a measure of the average diameter of typical space-filling polyhedra, which in moldavites are composed of SiO4 and AlO4 tetrahedra. This parameter results from the bond length and the ring size; for low-quartz a = 0.436 nm and for high-cristobalite a = 0.497 nm. Moldavite has a cristobalite-like intermediate structure with a ~ 0.485 nm (industrial silica glass a ~ 0.510 nm). The “f-value” characterizes the fluctuations of the intermediate range order. It monitors the “quality” of glass structure and can be calculated from the ratio of the “a-value” to the “correlation length”. In a homogenous and relaxed silicate glass f is ~ 0.35. Short fusion processes and fast cooling increase the “f-value”. For moldavites f ~ 0.46 and is greater than that of a relaxed glass. The value of a tending toward that of cristobalite correlates with the quenching of the glass from high temperature. The high value of the “f-value” correlates with a rapid cooling regime. The values of these parameters for moldavite suggest a short fusion process leading to high degree of structural and chemical disorder. Annealing of moldavites by heating up to 1000°C reduces the fluctuations and especially the mechanical stress.

Fig. 9.15. Micrograph of the surface of a moldavite specimen showing bubbles and strias resulting from rapid gas release.

The origins of tektites as rapidly quenched glasses from high temperature melts is further supported by the low degree of dissolved gases found in tektites. Mass loss measurements at atmospheric pressure show little mass loss up to 1600oC for Bohemian moldavites (Fig. 9.13). Additionally, due to the high alumina content, the glass is inhibited from devitrifying and, hence, no crystallisation takes place. In the TG experiments some mass loss is observed, but it is apparent that a similar mass loss is observed from both the first and second heating runs suggesting that mass loss is associated with adsorption of atmospheric gases at low temperature. This view is supported by the degassing experiments on these Bohemian moldavites. The degassing under vacuum shows even lower gas content and water content of the order of few ppm (0.0004 wt% (Fig. 9.14). The degassing experiments were carried out using a special high-vacuum-hot-extraction method DEGAS combined with a quadrupol mass spectrometer [57]. Analysis was performed using vacuum conditions (10-4 to 10-3 Pa), a linear heating rate (10K/min) from room temperature (RT) to 1450°C. The gaseous species were analyzed in multiple ion detection mode (MID) and correlated with the total pressure change in the heating chamber [58-60].

In contrast to degassing experiments using a Knudsen cell or capillary systems, the DEGAS experiments occur under highly non-equilibrium conditions. Under high vacuum, the reversibility of decomposition reactions producing volatile products is inhibited due to the rapid removal of volatile reaction products. Gas phase reactions are also inhibited due to the rapid removal of the gaseous products. The reaction products analysed are therefore more likely to be products of the primary decomposition or degassing process. Quantitative determination of volatiles is possible by calibration with crystalline materials e.g. for H2O from muscovite or CO2 from calcite decomposition. A typical resolution limit for water in glasses is approximately 1 ppm H2O. A gas content of 0.0004% fits within the resolution of the measurement [60]. The low volatile content of the moldavites is further confirmed in the micrograph shown in Fig. 9.15 where a number of bubbles and strias are observed as a consequence of rapid gas release during solidification.

9.4. Rare natural non-crystalline solids Moldavites and opal are rare natural non crystalline solids, whose primary value is aesthetic. Both material types are important both in their as found shapes, in the case of moldavite – the aerodynamic forms and colour and in the case of opal – the infilling of void seams, cracks and in fossil replacement, and as mineral gems for jewellery. Both of these natural “minerals” have an important part to play in the understanding of geological processes as their formation is a record of change in the natural environment [61,62]. The tektites, based on their models of formation have given an insight into the impact processes that have remodelled the Earth’s fauna and flora. The opal is the first step to understanding the diagenesis of silica and its mobility. Even though both of these non-crystalline solids are formed at the extremes of forming processes, both tektites and opals as forms of silica help to enrich our lives [50, 58].

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[61] Engelhardt W. v., Luft E., Arndt J., Schock H., Weiskirchner W. Origin of moldavites. Geochim.Cosmochim.Acta 51 1425 – 1443 (1987) [62] P. Thomas, J. Šesták, K. Heide, E. Füeglein and P. Šimon “Thermal Properties of Australian Sedimentary Opals and Czech Moldavites” Proc. Mediterian conf on TA, 2009

Thermal properties of Australian sedimentary opalsand Czech moldavites

P. S. Thomas • J. Sestak • K. Heide •

E. Fueglein • P. Simon

MEDICTA2009 Special Issue

� Akademiai Kiado, Budapest, Hungary 2010

Abstract The thermal properties are presented for a

precious opal sourced from Coober Pedy, South Australia

and a moldavite from Bohemia, Czech Republic whose

origins differ significantly as opal is derived from the slow

isothermal diagenesis of silica, while the tektites are

specimens of vitreous silica formed from the terrestrial

impact of asteroids. The differences between the two glassy

silicates are presented through measurement of the

TG–DSC, TMA and high-vacuum-hot-extraction DEGAS

analysis.

Keywords Amorphous Silica � DEGAS � Moldavite �Opal � TG–DSC � TMA

Introduction

Natural vitreous and diagenetic silicate glasses are found

widely distributed on the earth and lunar surfaces. The

vitreous glasses are formed from the melting and quench-

ing of silicate rocks, while the diagenetic process is

through the dissolution and precipitation of silica either

through chemical, hydrothermal, or biotic means. Although

both routes produce amorphous glassy substances, the

mechanism of formation results in glasses with differing

properties and in a range of quantities which can vary from

micrograms through to the kilo tonne quantities of the

‘‘glassy mountains’’ [1]. A range of thermal histories is also

observed. In the extremes, tektites like moldavites are

formed by extremely fast heating and melting at very high

temperatures ([3,000 K) followed by quenching at

extreme cooling rates (C106 K/s) [2]. In contrast, the

formation of glasses through mineral diagenesis or biotic

processes occurs at ambient temperatures; the formation

processes are essentially isothermal and take place over

long periods of time of the order of months to years or at

extremely low cooling rates (K/year) [3]. The formation

history of the natural glasses also results in a variety of

microstructures. All of these glasses are amorphous and as

such have no short range order, but under specific condi-

tions, long range order has been observed in the diagenetic

opaline glasses.

Although a variety of natural silicate glasses is available

for characterisation and discussion, this article limits

the discussion to two natural glasses, Moldavites from

Bohemia in the Czech Republic and sedimentary opal from

Coober Pedy in South Australia, as these glasses are found

on the microgram to gram scale, are desired for their

aesthetic qualities and are formed at the extremes of the

temperature and time profiles.

P. S. Thomas (&)

Department of Chemistry and Forensic Sciences,

University of Technology Sydney,

PO Box 123, Sydney, NSW 2007, Australia

e-mail: paul.thomas@uts.edu.au

J. Sestak

Solid-State Physics Section, Institute of Physics,

Academy of Sciences, v.v.i., Cukrovarnicka 10,

162 53 Praha 6, Czech Republic

K. Heide

Chemisch-Geowissenschaftliche Fakultat, Universitat Jena,

Burgweg 11, 07749 Jena, Germany

E. Fueglein

Netzsch-Geratebau GmbH, Wittelsbacherstraße 42,

95100 Selb, Germany

P. Simon

Department of Physical Chemistry, Faculty of Chemical

and Food Technology, Slovak University of Technology,

Radlinskeho 9, 812 37 Bratislava, Slovak Republic

123

J Therm Anal Calorim (2010) 99:861–867

DOI 10.1007/s10973-010-0706-z

Australian sedimentary opal

Precious or noble opal is prized for its play-of-colour

(POC), the origins of which are based on the diffraction of

visible light off ordered arrays of monodispersed silica

spheres [4, 5]. Precious opal is an unusual material as it is

composed of amorphous hydrous silica with the general

formula SiO2�nH2O, but is precipitated as monodispersed

colloidal spherical particles with diameters in the size

range 150 to 400 nm. The monodispersed colloid is con-

centrated by sedimentation, evaporation or by filtration

under pressure into ordered arrays that diffract visible light

to produce the characteristic, prized POC.

The monodispersed silica colloid is formed through the

dissolution and precipitation of silica. A likely source of

the silica is from the weathering of silicates [2, 3]. In the

weathering model, the source of the silica is the sedimen-

tary rocks associated with the GAB where the chemical

weathering of relatively soluble silicates such as the feld-

spars contained in these sediments results in the formation

of an alkaline silica solution. An example of such a mineral

is potassium feldspar which weathers through the idealised

stoichiometry:

2KAlSi3O8 þ 3H2O! Al2Si2O5 OHð Þ4þ2KOH þ 4SiO2

by the permeation of ground water through the sediments

resulting in kaolinite, dissolved silica and an increase in pH

through the release of potassium hydroxide. The reported

trace element distributions in opal from a wide variety of

sources are consistent with such a weathering model [6–8].

Once the silica is in solution, the enrichment of the solution

by evaporation or filtration can occur. Increasing the con-

centration of the silica solution coupled with a lowering of

the pH through alkali ion exchange with the surrounding

clays allows the nucleation of primary silica spheres and

subsequent sphere growth as more silica is supplied to the

system. The supply of silica in the solution also appears to

be a cyclic process as generations of growth rings are

observed in the silica spheres. Once the monodispersed

silica spheres have reached a suitable size, concentration of

the colloid is then required and in certain cases, ‘crystal-

lisation’ of the monodispersed colloid occurs to form the

ordered arrays which results in the prized POC. The

interstices are subsequently in-filled with a silica cement

completing the formation of the opal producing a hard

material which, despite the gel-like structure, has a Ber-

kovich hardness of 5.7 to 6.2 GPa which is similar to that

of soda glass (6.4 GPa), but, as might be expected for a gel

like material, is significantly less than that of fused silica

(10.8 GPa) [9].

Although the weathering model is important in aiding

the understanding of the formation of opal, a number of

other models have been proposed to account for specific

observations in specimens acquired by the authors of these

models. Examples of such models are the ‘‘microbial’’

model where microbes have been observed in Lightning

Ridge matrix specimens suggesting that microbial action is

responsible for the source of the silica [10], the syntech-

tonic fluid model where high pressure, warm hydraulic

silica rich fluids are the silica source [11] and the mound

spring model where the silica rich alkaline waters derived

from the artesian basin well up through mound springs

supplying the silica rich solutions required for opal for-

mation [12]. Although these models source the silica from

different origins, the formation of the monodispersed

colloid must, however, in each case occur by homogenous

precipitation followed by a concentration mechanism such

as evaporation, filtration or sedimentation.

A sedimentation mechanism for the formation of opal

does have some support [8, 13]. Elemental distributions in

banded opal have suggested that although the bands must be

formed from the same solution (e.g., constant aluminium

concentrations across bands), the variation in ion concen-

tration between bands of highly charged metal ions suggests

that the bands form by flocculation of the silica colloid.

Leisegang type phase separation (repeating colour rings) on

precipitation of the silica may also explain the concentration

variations of trace elements between bands [14].

The ‘sol–gel’ nature of the formation of opal is respon-

sible for the hydrous gel-like structure of the silica and as a

consequence opal contains a significant proportion of water

much of which is reported to be molecular [15, 16]. Four

types of water have been postulated: molecular water surface

adsorbed (e.g. pore), bulk cage molecular water, surface

silanol water and bulk silanol water [3]. Quantification of the

amount of each species has proven to be difficult; however,

Langer and Florke [16] from their infrared spectroscopic

studies suggested that approximately 90% of the water

contained in sedimentary opal was present as molecular

water which confirmed Segnet et al’s [15] postulation based

on the calculation of surface area of the spheres that com-

prised POC opal. Further confirmation has been supplied by29Si NMR measurements where approximately 10 and 26%

of the water was estimated to be in the form of silanol groups

in Coober Pedy white POC opal and Tintenbar crystal opal,

respectively [14, 17]. Thermal analysis has also demon-

strated the presence of a significant proportion of molecular

water in both volcanic and sedimentary opal [18].

Tektites and moldavites

Tektites are centimetre to decimetre-sized bottle green to

blackish glassy ‘‘bodies’’. Tektite is derived from the

862 P. S. Thomas et al.

123

Greek, tektos meaning molten and tekein to melt. They are

found in gravels ranging in age from upper tertiary to

alluvial in the Radomilice area, Chlum near the Moldau

river in Moravia, Bohemia, Czech Republic (Moldavites),

in Lusatia, Saxonia Germany, in Indochina, Vietnam,

Thailand, Malayan Peninsula, Java, Flores, Billiton,

Borneo, Philippines, Australia and Tasmania (Australites),

in Zhamnanshin (Irghizites) Kazakhstan, Ouellee on the

Ivory Coast in Africa, Texas and Georgia in the United

States (Bediasites). Microtektites (vitreous/crystalline

spherules \500 lm) have been found in deep-sea deposits

in the Gulf of Mexico, Caribbean Sea, north-western

Atlantic Ocean and on Barbados. The chemical composi-

tion of microtektites is closely associated with the North

American tektites. Transparent colourless to pale brown

microtektites are also found associated with clinopyroxene

spherules in the western Pacific and Indian Ocean in the

upper Eocene marine deposits [19].

The origin of tektites was intensively studied and dis-

cussed in the 20th century, particularly in the latter

decades, with respect to the mass extinction during the

short time event known as the Cretaceous–Tertiary

boundary. The discussion was dominated by two alter-

native origins of tektite material: the Terrestrial Impact

Theory (TIT) and the Lunar Volcanic Theory (LVT) [20,

21]). As Izokh [20] noted the TIT ‘‘won a complete

victory over O’Keefe’s [21] Lunar Volcanic Theory’’ and,

currently, the majority of scientific community discusses

the origin of tektites and mass extinction event on the

basis of TIT as, in particular, the chemical evidence

supports the formation of tektites from terrestrial rocks

and soils [22].

The terrestrial impact theory is based on the significant

transfer of energy from impacting asteroids with the earth’s

surface. As asteroids are massive, impact with the earth

surface produces high-energy collisions and high pressures

and temperatures in the impact zone which results in the

fusion and vaporisation of the material around the impact

zone. The energy of these impacts was such that fused and

vaporised material was catapulted out to near space. As this

material returned to earth, it rapidly cooled and solidified

with an absence of gaseous inclusions (although vacuous

inclusions are observed). During the quenching process the

solidifying phase was generally shaped aerodynamically

and as the molten droplets were spinning, a range of bizarre

shapes; plate, spheroid, rod, dumbbell, teardrop, star, curly,

winkled and bubble, were produced. As these particles

were returned to earth, the particles were distributed by

aerodynamic transport over large areas on the Earth-

surface in strewn fields. The tektites found in particular

strewn fields are postulated to have been derived from the

same impact event due to the high degree of compositional

homogeneity within an individual strewn field [23].

The impact-formed or metamorphic glasses have a sig-

nificant durability. The age of impact-formed glasses at the

earth’s surface has been determined to be between

700,000 years (australites) and 34 million years (bediasites).

In a more inert environment such as at the lunar surface, glass

spherules have been aged at more than 2 billion years. In

general, recovered specimens of tektite have been observed

to be resistant to hydration or devitrification of the bulk [24].

Only a characteristic corrosion of the surface is observed at

the Bohemian tektites—the moldavites [25].

Experimental

The opal samples investigated were sourced from the Shell

Patch field in Coober Pedy, South Australia. The moldavite

samples were sourced from Bohemia in the Czech Republic.

Thermomechanical analysis (TMA) was carried out on

rectangular logs of opal (5 9 5 9 10 mm) and moldavite

(2 9 3 9 5 mm) using a TA Instruments 2940 Thermo-

mechanical Analyser by heating the specimens to 960 �C at

a heating rate of 1 �C/min in a static air atmosphere

followed by cooling at 5 �C/min to room temperature. The

cycle was then repeated for each sample.

Density measurements were carried out on the TMA

specimens before and after the heating cycles using the

Archimedes’ principle; qsample = (qliquid 9 dry weight)/

(dry weight - immersed weight). The immersed weight

was determined by immersion in analytical grade hexane.

The density of the hexane was measured using a

pycnometer of known volume.

A direct-coupled-evolved-gas-analysis-system (DEGAS)

was used to carry out a high-vacuum-hot-extraction of the

evolved gases with gas analysis carried out using a quadrupol

mass spectrometer [26, 27]. Pieces of opal and moldavite of

the order of 100 mg were heated at a rate of 10 �C/min under

vacuum (10-4 Pa) to 1,400 �C, while the release of water

(m/z = 18 amu), hydrogen (m/z = 2 amu) and methane

(m/z = 13 and 15 amu) were monitored.

Simultaneous thermogravimetric analysis (TG)–differ-

ential scanning calorimetry (DSC) was carried out on a

Netzsch STA 449 C Jupiter by heating 40–60 mg of sample

in a platinum crucible at 20 �C/min in a synthetic air

atmosphere (N2:O2 80:20) at a flow rate of 70 mL/min to

1,640 �C.

Results

Thermogravimetric analysis–differential scanning

calorimetry (TG–DSC)

Simultaneous TG–DSC data for the opal and the moldavite

specimens are shown in Fig. 1. The measurements are

Thermal properties of opals and moldavites 863

123

characteristic of the origins of these specimens. As the opal is

formed though a solution-precipitation process, a significant

amount of water (7.5% by weight) is contained within the

specimen which is removed between 200 and 1,000 �C, with

peak rate of loss from the DTG curve (not shown) at 297 �C.

The second heating curve shows, as expected, no mass loss.

As the Moldavite was formed through a process of rapid

melting to high temperature followed by rapid cooling, no

mass loss was expected. A small mass loss of 0.39 and 0.34%

in the 1st and 2nd heating runs, respectively, was, however,

observed (Fig. 1b). As this small mass loss was reproduced

in both heating runs, the mass loss was ascribed to the

removal of gases adsorbed at low temperature which diffuse

into the pore structure and, hence, are removed slowly over

the period of the heating.

The 1st heating run DSC curve for the opal (Fig. 1a)

shows an endotherm at 303 �C which corresponds to the

maximum water loss. At elevated temperature, an exo-

thermic peak around 1,287 �C with an onset at 1,166 �C is

observed. Reheating the specimen in the 2nd heating run

reveals an endothermic peak with an onset temperature of

211 �C and a peak temperature of 217 �C that corresponds

to the a- to b-cristobalite transition. The exotherm above

1,166 �C in the 1st heating curve can, therefore, be ascri-

bed to crystallisation of the opal. The DSC curves for the

moldavite are essentially featureless showing that even

Heat flow

/W/g

Heat flow

/W/g

Mas

s/%

Residual mass: 92.5%exo

TG 2nd heating

TG 1st heating

217°C303°C

1166°C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

99.5

99.6

99.7

99.8

99.9

100106

104

102

100

98

96

94

920 500 1000 1500

–2

–1

0

1

0 500 1000 1500

Mas

s/%

Temperature/°CTemperature/°C

DSC 1st heating

DSC 2nd heating

DSC 2nd heating

DSC 1st heating

TG 2nd heating

TG 1st heating

exo

(a) (b)

Fig. 1 TG and DSC data of the CP white play-of colour opal (a) and Bohemian moldavite (b) heated to 1640 �C at 20 K/min in an air

atmosphere

1050 1100 1150 1200 1250 1300 1350 1400–2.0 x 10–11

0,0

2.0 x 10–11

4.0 x 10–11

6.0 x 10–11

8.0x 10 –11

1.0 x 10–10

0,0004 wt% H2O

Fläche Zentriert Höhe---------------------------------------------1,26943E-9 1266,79833 9,55971E-11

Water releaseMoldavite 2122,5 mg

Ion

curr

ent/Å

Temperature/°C Temperature/°C0 200 400 600 800 1000 1200 1400

0,0

5.0 x 10–9

1.0 x 10–8

1.5 x 10–8

2.0 x 10–8

200°C

1219°C1024°C

689°C

456°C

Opal Coober PedyAustralian3 mg

Water release

Ion

curr

ent m

/z 1

8

(a) (b)

(c) (d)

0 200 400 600 800 1000 1200 14000.0

5.0 x 10–9

1.0 x 10–8

1.5 x 10–8

2.0 x 10–8Opal Coober PedyAustralian

(2)

(1)

Water release (1)

Ion

curr

ent/Å

Temperature/°C0 200 400 600 800 1000 1200 1400 1600

Temperature/°C

0,0

5,0 x 10–11

1,0 x 10–10

Hydrogen release (2)

0.0

1.0 x 10–12

2.0 x 10–12

3.0 x 10–12

4.0 x 10–12

5.0 x 10–12

6.0 x 10–12

7.0 x 10–12

0.0

2.0 x 10–11

4.0 x 10–11

6.0 x 10–11

8.0 x 10–11

1.0 x 10–10

1.2 x 10–10

1.4 x 10–10

m /z 15Opal Coober PedyCH (m/z 13)-release

m e13

Ion

curr

ent/Å

Methane -release

Fig. 2 DEGAS data for a the water evolution from a moldavite specimen and b the water, c the hydrogen and d the methane evolution from a

Coober Pedy white POC opal

864 P. S. Thomas et al.

123

heating to 1,640 �C had no effect on the morphology of the

specimen which is likely to be due to the relatively high

alumina content of moldavites (10.3% [23]).

High-vacuum-hot-extraction DEGAS

The DEGAS data is displayed in Fig. 2. Negligible gas

evolution was observed from the moldavite (Fig. 2a) with

only 0.0004% mass loss due to water evolved above

1,200 �C. The mass loss ascribed to the adsorbed gases

under atmospheric conditions observed in the TG data are

not present as this technique is carried out under vacuum

thus removing any adsorbed gasses prior to elevation of the

temperature.

The thermal extraction of gases from the opal, however,

is much more eventful. Figure 2b shows the DEGAS curve

for water which corresponds to 99.6% of the total mass

loss. The evolution of water is observed as a main peak

around 200 �C which is lower than that observed in the TG

data. The DEGAS is carried out under the reduced pressure

which shifts the peak to lower temperature. A shoulder to

the main evolution peak is also observed between 400 and

700 �C accounting for 0.3% of the water loss and contin-

ued evolution of water is observed up to and beyond

1,000 �C accounting for 0.1% of the water loss and is

consistent with the TG data. It is also notable that the

spikes in the data are observed. These spikes correspond to

sudden losses which may be ascribed to sudden release

from fluid inclusions or entrapped micropores in which the

pressure escalates to beyond the breaking stress of the opal.

Figure 2c and d show the DEGAS curves for hydrogen

and methane, respectively. It should be noted that the

combined evolution of these species corresponds to less

than 0.4% of the total mass loss. The hydrogen released

around 880 �C is likely to be derived from micellar

decomposition of silanols [28], but the methane evolved

around 100 �C is a little more difficult to explain as the

temperatures are too low for the micellar reactions inferred

for the evolution of hydrogen. It is possible that very small

proportions of molecular methane are trapped in the gel

structure as the opal network is formed.

Thermomechanical analysis (TMA)

The TMA data for the opal and moldavite is shown in

Fig. 3. The data for opal is characteristic of a hydrated

silica where, in the 1st heating, significant expansion

occurs up to 210 �C. Above 210 �C, contraction occurs

which is associated with the densification of the silica

network on removal of the water as well as sintering of the

structure [18, 29]. At 540 �C, the onset of a second

expansion is observed for this sample of opal and is

ascribed to a ‘blowing’ of the structure due to the increased

pressure of water contained within the structure. It is also

possible that this second expansion can be ascribed to slow

cracking of the opal which can occur and was observed in

the specimen after removal from the furnace. The expan-

sion is, however, reproducible for samples which undergo

this second expansion although it should be noted that not

all samples of opal undergo this type of expansion; many

samples sourced from different fields in Coober Pedy or in

other regions do not undergo this type of expansion [18].

Further heating results in contraction as further sintering

and densification of the structure occurs [29]. The second

heating of the opal produces a linear expansion curve that

might be expected of a dehydrated specimen. The molda-

vite specimens produced reproducible expansion curves

each time a specimen was heated suggesting that heating

the moldavite to 960 �C did not significantly affect its

structure which is in keeping with the TG data and reported

expansion measurements of tektites [2].

The expansion data for the opal and moldavite speci-

mens are listed in Table 1. Data is listed so that comparison

can be made between the samples in the linear regions of

the expansion curves. Not surprisingly, the greatest

–1.2

–1.0

–0.8

–0.6

–0.4

–0.2

0.0

0.2

0.4

0.6

0.8

0 200 400 600 800 1000

Dim

ensi

on c

hang

e/%

/°C

Temperature /°C

Coober Pedy Opal 2nd heating

Coober Pedy Opal 1st heating

Moldavite 2nd heating

Moldavite 1st heating

Fig. 3 TMA data for 1st and 2nd heating runs on a moldavite

specimen and a Coober Pedy

Table 1 Expansion Coefficients determined for heating and cooling

curves using TMA for Coober Pedy POC opal and Czech Moldavite

Sample Heating

100–200 �C

(910-6/�C)

Heating

300–500 �C

(910-6/�C)

Coober Pedy 1st heating ?7.4 -13.6

Coober Pedy 2nd heating ?1.4 ?1.3

Moldavite 1st heating ?3.7 ?4.6

Moldavite 2nd heating ?4.3 ?4.1

Thermal properties of opals and moldavites 865

123

expansion coefficient is observed for the opal on 1st

heating and is consistent with the water exerting hydro-

static pressure on the opal causing significant expansion.

However, once dehydrated, the opal specimen has the

lowest expansion coefficient. Although opal contains

approximately 99% SiO2 on an anhydrous basis, the

expansion coefficient for dehydrated opal remains signifi-

cantly greater than that of fused silica (0.5 9 10-6/oC)

[2, 24]. The expansion coefficient for moldavite is greater

than that for dehydrated opal, but is less than that for

obsidian (6.3 9 10-6K-1) and soda lime glass (7.5 9

10-6K-1) [2, 24]. The thermal expansion coefficient for

moldavite is most aptly compared to that of obsidian as

both minerals have a similar rhyolitic composition. The

difference in the expansion coefficient is most likely due to

the relative proportions of sodium and potassium with

respect to calcium and magnesium within the silica net-

work. Opal has approximately 0.5% alkali and alkali earth

metals as part of its composition thus raising the expansion

coefficient to above that for fused silica. Moldavite con-

tains significantly greater concentrations of the alkali and

alkali earths and thus the expansion coefficient is greater

than that of the dehydrated opal. Although moldavites and

obsidian have a similar rhyolitic composition, moldavites

have a greater concentration of CaO and MgO and lower

Na2O content than obsidian which results in a more tightly

bound structure and, hence, a lower expansion coefficient

[2].

The densities for the TMA specimens before and after the

cycling are listed in Table 2. The density of the opal

increases, as might be expected, as sintering of the specimen

occurs at elevated temperature [18, 29, 30]. This change in

density is, however, small and indicates that the structure has

not significantly changed during heating to 960 �C. The

small change in density is also indicative of the nature of the

water in opal; that is, molecular water trapped in silica cages.

The change in density of the moldavites is, however,

somewhat surprising as a reduction in the density after

heating in the TMA is observed. This is in contravention to

published data [2]. It is likely that this reduction in density is

due to the thermal history as the samples investigated were

twice heated to 960 �C at 1 �C/min and cooled under con-

trolled conditions at a cooling rate of 5 �C/min allowing

relaxation of the glass to a marginally more open structure.

Discussion

Opal and moldavite were selected for comparison in this

study as their formation histories are at the extremes. The

extreme conditions under which the moldavites are pro-

duced results in a vitreous glass which is completely

degassed and, therefore, shows no mass loss either in the

TG or in the DEGAS experiments where little evolution of

volatiles was observed. The opal, by contrast, is a solution

precipitated glassy substance and this sample has a water

content of 7.5% (some variability in the water contents in

opal are observed and depends on the field within a region

as well as the level (depth) within a mine). The predomi-

nant mass loss for this sample is associated with the water

loss (99.6%). Hydrogen was observed to be evolved at

elevated temperature but this was ascribed to cage

decomposition of the silanol groups. Methane was also

observed and was observed to evolve at low temperature

suggesting that molecules of methane are trapped in silica

cages. For the methane to be trapped in the silica cages, the

methane must have been present in the silica solution as the

opal was formed. The origin of the methane is unclear, but

it may have originated from the artesian waters of the Great

Artesian Basin or from meteoric waters.

Expansion coefficients for the as received samples are

consistent with the relative contents of volatiles (water)

present in the opal and the moldavite. The moldavite is

unaffected by the heating to 960 �C except for a small

decrease in density. The opal is dehydrated and although

cracking occurs during the first heating, subsequent heating

cycles produces linear and reproducible expansion curves

with coefficients of expansion less than that of moldavite,

but greater than that of fused silica which is consistent with

the relative elemental compositions. The small increase in

density after the heating cycle indicates that the silica

structure in the opal is also resistant to rearrangement

below 1,000 �C. Above 1,166 �C, however, crystallisation

to cristobalite is observed whereas the moldavite is

apparently unaffected by heating to 1,640 �C. These dif-

ferences may be attributed to the differences in the alumina

content which is of the order of 1% for the opal and of the

order of 10% for the moldavites.

Final remarks

Opals and moldavites are rare natural non-crystalline

solids, whose primary value is aesthetic. Both are impor-

tant both in their as found shapes: in the case of moldavite

the aerodynamic forms and colour and in the case of opal

the infilling of voids, seams, cracks and in fossil replace-

ment and both are valued as mineral gems for jewellery.

Table 2 Density measurements for Czech moldavite and Coober

Pedy POC opal before and after heating to 960 �C in the TMA

Sample Density (g/cm3)

Coober Pedy Opal before heating 2.109

Coober Pedy Opal after heating to 960 �C 2.115

Moldavite before heating 2.375

Moldavite after heating to 960 �C 2.354

866 P. S. Thomas et al.

123

Both of these natural minerals have an important part to

play in the understanding of geological processes as their

formation is a record of change in the natural environment.

The tektites, based on their models of formation, have

given an insight into the impact processes that have

remodelled the Earth’s fauna and flora. The opal is the first

step to understanding the diagenesis of silica and its

mobility. Even though both of these non-crystalline solids

are formed at the extremes of forming processes, both

tektites and opals help to enrich our lives.

References

1. Heide K. Geheimnisse des ‘‘Glasernen Bergs’’. Die DFG-fors-

chung. 2007;3:24–7.

2. Heide K, Heide G, Kloess G. Glass chemistry of tektites. Planet

Space Sci. 2001;49:839–44.

3. Iler RK. The chemistry of silica, solubility, polymerisation, col-

loid and surface properties, and biochemistry. New York: Wiley;

1979.

4. Jones JB, Sanders JV, Segnit ER. The structure of opal. Nature.

1964;4962:990–1.

5. Darragh PJ, Gaskin AJ, Sanders JV. Opals. Sci Am. 1976;234

(4):84–95.

6. McOrist GD, Smallwood A. Trace elements in precious and

common opals using neutron activation analysis. J Radioanal

Nucl Chem. 1997;223(1–2):9–15.

7. Erel E, Aubriet F, Finqueneisel G, Muller JF. Capabilities of laser

ablation mass spectrometry in the differentiation of natural and

artificial opal gemstones. Anal Chem. 2003;75(23):6422–9.

8. Brown LD, Ray AS, Thomas PS. Elemental analysis of Austra-

lian amorphous opals by laser-ablation ICPMS. Neues Jahrbuch

fur Mineralogie Monotshefte. 2004;2004(9):411–24.

9. Thomas PS, Smallwood AS, Ray AS, Briscoe BJ, Parsonage D.

Nanoindentation hardness of banded australian sedimentary opal.

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074028.

10. Behr HJ, Behr K, Watkins JJ. Cretaceous microbes—producer of

black opal at Lightning Ridge, NSW, Australia. Geological

Abstracts No. 59. In: 15th Australian Geological Convention,

Sydney, July; 2000.

11. Pecover SR. A new genetic model for the origin of precious opal.

Extended abstracts no. 43. In: Mesozoic geology of Eastern

Australia plate conference, Geological Society of Australia; 1996.

pp. 450–454.

12. Devison B. The origin of precious opal—a new model. Aust

Gemmol. 2004;22(2):50–8.

13. Brown LD, Thomas PS, Ray AS, Prince K. A SIMS study of the

transition metal element distribution between bands in banded

Australian sedimentary opal from the Lightning Ridge locality.

N Jb Miner Monotshefte. 2006;182(2):193–9.

14. Brown LD. Characterisation of Australian opals. PhD Thesis,

University of Technology Sydney; 2005.

15. Segnit ER, Stevens TJ, Jones JB. The role of water in opal. J Geol

Soc Aust. 1965;12(2):211–26.

16. Langer K, Florke OW. Near infrared absorption spectra (4000–

9000 cm-1) of opals and the role of water in these SiO2�nH2O

Minerals. Fortschritte der Mineralogie. 1974;52(1):17–51.

17. Brown LD, Ray AS, Thomas PS. 29Si and 27Al NMR study of

amorphous and paracrystalline opals from Australia. J Non-Cryst

Solids. 2003;332:242–8.

18. Smallwood AG, Thomas PS, Ray AS. Thermal characterisation

of Australian sedimentary and volcanic precious opal. J Therm

Anal Calorim. 2008;92(1):91–5.

19. Glass BP. Tektites. J Non-Cryst Solids. 1984;67:333–44.

20. Izokh EP. Origin of Tektites: An Alternative to Terrestrial Impact

Theory. Chemie d.Erde. 1996;56:458–74.

21. O’Keefe JA. Tektites and their origin. Amsterdam: Elsevier;

1976.

22. O’Keefe JA. Natural glasses. J Non-Cryst Solids. 1984;67:1–17.

23. Lange JM. Lausitzer Moldavite und ihre Fundschichten. Schrif-

tenr.f.Geowiss. 1995;3:7–138.

24. Heide K, Kletti H. Resistance of natural glass. Glass Sci Technol.

2003;76:118–24.

25. Bouska V. Natural glasses. Praha: Academia; 1993.

26. Schops D, Schmidt CM, Heide K. Quantitative EGA analysis of

H2O in silicate glasses. J Therm Anal Calorim. 2005;80:749–52.

27. Heide K, Gerth K, Hartmann E. The detection of an inorganic

hydrocarbon formation in silicate melts by means of a direct-

coupled-evolved-gas-analysis-system (DEGAS). Thermochim

Acta. 2000;354:165–72.

28. Heide K, Woermann E, Ulmer G. Volatiles in pillows of the

mid-ocean-ridge-basalt (MORB) and vitreous basaltic rims.

Chemie der Erde. 2008;68:353–68.

29. Thomas PS, Smallwood A, Ray AS, Simon P. TMA and SEM

characterisation of the thermal dehydration of australian sedi-

mentary opal. J Thermal Anal Calorim. 2007;88(1):185–8.

30. Smallwood AG. Chemical and physical evaluation of Australian

precious opal. MSc Thesis, University of Technology Sydney; 1999.

Thermal properties of opals and moldavites 867

123

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 365–373

ON THE CHRONICLE OF HIGH-Tc OXIDESUPERCONDUCTORSInterdisciplinary of thermochemistry, stoichiometry andthermodynamics

J. Šesták* and P. LipavskýAcademy of Sciences, Institute of Physics, Cukrovarnická 10, CZ - 162 53 Prague, CzechRepublic

Abstract

After briefly noting some development records in phase diagrams, the stoichiometry is dealt with inmore details showing the structural layer ordering in the superconducting cuprates and mentioningsome correlation between thermodynamics and BSC theory of superconductivity.

Keywords: cuprates, stoichiometry, superconductivity, thermodynamics

Introduction

In the 1990s the thermochemical research was the main engine in the boom progress ofnew sort of superconducting materials called the high Tc superconductors (HTSC). Spe-cialized thermoananlytical studies at low temperatures became a useful tool in depictingcharacteristic behavior. The crucial impulse was the surprising discovery that a ceramicsample, i.e., barium-doped lanthanum copper oxides become superconducting at 36 K,some 12 K above the previous record temperature for traditional transition metals alloys.The subsequent quest for the other promising cuprates yielded materials with transitiontemperature (Tc) far above the boiling temperature of nitrogen (77 K) so that they couldbe used with cheaply available coolant rather than expensive liquid helium. Although theresearch boom is gradually expiring the commercial applications have been too slow totake off and the early promise of magnetically-levitated trains, powerful electric motorsor super-efficient power transmission has not been met.

A major difficulty with the ceramic HTSC is that such complex metal oxideshave high mechanical fragility so its ability to bend is severely limited. Although thishas been overcome by surrounding its filaments with the highly pliable metals (Ag)there is also a ceiling on the amount of current that can be carried and another level ofmagnetic field in which the system can be operated. Other effort to bond cuprate su-perconductors to substrates has a target: melt and liquid coating methods, in the hope

1388–6150/2003/ $ 20.00

© 2003 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest

Kluwer Academic Publishers, Dordrecht

* Author for correspondence: E–mail: sestak@fzu.cz

to cut the cost. Most progressive seems to be the growth of buffer layer directly onto aNi-alloy where intermediate NiO layer exhibits good compatibility with HTSC. Sucha liquid phase epitaxy (LPE), with no need of high-vacuum components, is cheaper tobuild as compared with traditional chemical vapor deposition (CVD) and provideshigher growth rates. Electrical engineers are looking forward to some of such pro-cesses that can help reducing the capital and operating costs of using HTSC. For ex-ample, unlike conventional large copper wire-based power transformers, which arecooled by oil and have to be located outside on hard standings massive enough to sup-port their weight, HTSC equivalents would use liquid nitrogen, which is widely avail-able commodity, cheap enough and intrinsically secure making available muchlighter units so that they could be installed indoor safely, near to the point of poweruse. The process of entire HTSC applicability seems to replicate another case of me-tallic glasses, equally innovative and promising material of seventies, that never getactually in practical use in transformers due to unanswered intricacy of their thermo-dynamic stability. In that case thermochemical research contributed most influentialfunction, too. Nevertheless there is the first important HTSC application, worth men-tioning, as a security switch in high power electric circuits, where the massive rod ofHTSC is coupled in-between and kept near the critical current state to secure short-cut’s explosions that often happen when using classical fusses.

We are happy to mention that in our mother Institute of Physics we took earlypart in the worldwide development of HTSC having been interested in various as-pects of structural and preparative thermochemistry, thermodynamics and even tech-nology of phases as well as, more recently, in certain theoretical aspects of supercon-ductivity. Among others the investigations near the beginning were: the characteriza-tion of famous phase, YBa2Cu3Ox, abbreviated as (111) [1], initial studies inquenched (111) [2] (which we, unfortunately, did not completed to develop practicalutilization for HTSC wire texturing while drowned by means of fast melt solidifica-tion and glass-pattern recrystallization [3–5]), early description of kinetic problemswith the (111) phase formation and decomposition [6], etc., not mentioning thegrowth of single crystals and various methods of powder technologies.

It is clear that thermochemical research played a decisive role so that it became alsoreflected in extended publication activity in relevant journals, namely J. Thermal Analy-sis and Thermochimica Acta and herewith published special issues [7, 8]. The progress ofHTSC has been always associated with better understanding of phases and phase dia-grams starting from the oxide (Cu, Ba, Y), their binaries [9, 10] through pseudobinariesup to the Y–Ba–Cu–O pseudoternaries [9–14]. We paid increased attention to the im-provement of calculation of phases and their thermodynamic data [15, 16], the construc-tion of phase diagrams, which also proceeded with the novel families ofBi–Ca–Sr–Cu–O [17, 18] and Hg–Ca–Ba–Cu–O [19, 20] systems. More than ten yearsexperience with the construction of various types of phase diagrams and investigationprocedures (to estimate associated thermodynamic data) were summarized in our previ-ous reports [12–14, 21, 22]. We put in use a specific representation depicting the behav-ior of volatile phases (O2, Hg) enabling construction of so-called chemical potential dia-grams in two- and three-metal system, portrayed in Fig. 1 [23].

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In contradiction to the structural models we could hypothetically account just for thestoichiometric existence of various phases yet unknown by a formal extending the cationratios to ranges of yet undetermined compounds but suitable for thermodynamic calcula-tions. Such hypothetical changes of ratios (Y+Ba)/Cu (from <1, through ≈2 to >2.5) canbe derived, on one side, from the basic superconductor YBa2Cu3Ox, and on the other side,from the non-superconducting ‘green phase’ Y2BaCuOx (211). We can also extract par-tial values for the formation enthalpy along such homological series. For example, ac-cording to the stoichiometry changes along

YBa2Cu3O7 → YBa2Cu4O8 → YBa2Cu5O9 → (1/2)Y2Ba4Cu7O15

we can estimate that each adding of a single CuO conveys 19.8 kJ mol–1, while thatfor Cu2O is 12.13 kJ mol–1 and the multiplication of BaO layers provides the valuebetween 29–32 kJ mol–1 [12].

By using a particular thermoanalytical technique a special feature of the HTSCbehavior was elucidated, such the emanation thermal analysis in determining oxygenchanges in Ba-cuprates [6, 24] or sorptometric measurements aimed to analyze thesurface heterogeneity of Hg-cuprates showing its fractal-like dimensions [25]. In thisreport, however, we want to concentrate to HTSC as viewed from yet another cross-disciplinary aspects of thermochemistry, thermodynamics and thermal physics as amatter of interest for some involved thermoanalysts.

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���������� ���: HIGH-Tc OXIDE SUPERCONDUCTORS 367

Fig. 1 Account on plausible phases and their stoichiometry to occur in the systemBa–Ca–Cu–Hg–O. It is illustrated by the three-dimensional plot of chemical po-tential (µ) for three metals involved (Ba–Ca–Cu) in the conservative stage (c)assuming two volatile components (O and Hg) at the temperature T=773 K andpressure P=1 atm. The re-reproduced computer print-out was provided by thecourtesy of Prof. H. Yokakawa (Tsukuba, Japan)

Stoichiometry and superconducting state

Superconductivity, one of the most fascinating phenomena in solid-state physics wasdiscovered in 1911 by Kamerlingh Onnes [26] but it was until 1957 that a satisfactorymicroscopic theory of the effect became available, i.e., one by Bardeen, Cooper andSchieffer (BBC) [27]. The special correlation responsible for superconductivity arepair correlations, which in the presence of electron attraction leads to the formation ofelectron pairs, often called Cooper pairs [28]. Each pair can be considered as a boundstate of two electrons with opposite spin and momentum and may form even if the in-teractions are repulsive due to certain stringent requirements (i.e., the interactionsmust be much less repulsive for electrons near the Fermi surface than away from itand electron pairs can be treated as being independent of each other).

Impurity atoms in a material are usually viewed as a problem because they can re-sult in properties deviating from those ideally assumed. However, they can be sometimesused to advantage when attempting to understand new complex materials. This is be-cause the interaction of such doping atoms with matrix can reveal detailed information onthe local electronic environment, which is the case of HTSC. Here the electron–electronattractive interactions occur in the degenerated electron gas as a result of the formation ofabove-mentioned Cooper-pairs. The superconducting ground-state represents ‘condensa-tion’ of these Cooper-pairs into a single macroscopic quantum state – a process analo-gous to the fundamental Bose–Einstein condensation. The order parameter associatedwith this ground-state has d-wave symmetry. The examples of the HTSC materials areshown in Table 1. Whereas in these systems the conduction is due to holes in the another,electron-doped system of Nd2–xCexCuO4 (Tc ≅ 20 K) this is different.

The common structural features of all HTSC is a perovskite network MMeO3

[29] formed by densely coordinated oxygens being the source of two sublattices, onewith the large M (≡Ba, Sr) cations with 12-coordination and small Me (≡Cu) cationswith 6-coordination. The most important structural element are the copperoxideplanes with a unit cell CuO2, which are formed from octahedral, pyramids or squares.Examples are La2–xSrxCuO4, where the planes are formed from octahedral,YBa2Cu3O7 from pyramids and Nd2–xCexCuO4 from squares. The electron transportand the physical processes leading to superconductivity are believed to take placewithin these planes. In the simplest case of LaCuO4 the CuO6 octahedra are elongateddue to the Jahn–Teller distortion, lifting the degeneracy of the Cu–d orbitals, which istwo- and three-fold. This leaves for Cu a valency 2+ implying a 3d 9 configuration.The hole in the 3d shell is placed into the highest anti-bonding Cu–O state, which haspredominantly 3dx2–y2 character. With one hole per formula unit of LaCuO4, one canexpect its metallic character with a half-filled conduction band. La2CuO4 can bedoped with holes by partial replacing La3+ by Sr2+, similarly Nd2CuO4 can be dopedwith electrons by the replacement of Nd3+ with Ce3.5+. The Cu–O planes ofYBa2Cu3O7 contains holes without further modification because the charge is movedfrom the planes into the chains (so-called self-doping), which are formed from CuO4

squares sharing an oxygen atom.

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Table 1 Structural layer ordering in the superconducting cuprates and their critical temperaturesfor n=1, 2, 3 and 4

Superconducting cupratesCritical temperature/K for

n=1 n=2 n=3 n=4

Y-cuprates 93 80 – –

YBa2Cu2+nO5+n+δ

⇓ δ ⇑ e–

[CuOδ]2(1–δ)+ [BaO]0 [CuO2]

2– [Y]3+ [CuO]2– [BaO]0 [CuO]2(1–δ)+

Bi-cuprates 12 85 108 –

Bi2Sr2Can–1CunO2n+4+δ

⇓ δ ⇑ e–

[BiO]+ [BiO]+ [SrO]0 [CuO2]2– [Ca]2+ [CuO2]

2– [SrO]0 [BiO]+ [BiO]+

Tl-cuprates 95 118 125 –

Tl2Ba2Can–1CunO2n+4+δ

⇑ Tl ⇑ e–

[TlO]+ [TlO]+ [BaO]0 [CuO2]2– [Ca]2+ [CuO2]

2– [BaO]0 [TlO]+ [TlO]+

TlBa2Can–1CunO2n+3+δ

⇑ δ ⇓ e–

[TlO]+ [BaO]0 [CuO2]1.66– [Ca]2+ [CuO2]

1.66– [BaO]0 [TlO]+

Hg-cuprates 98 120 135 118

Hg2Ba2Can–1CunO2n+2+δ

⇓ δ ⇑ e–

[HgOδ](2–2δ)+ [BaO]0 [CuO2]

2– [Ca]2+ [CuO2]2– [BaO]0 [HgOδ]

(2–2δ)+

Among the most suitable cuprates are those fulfilling relation, dM–O ≤√2dMe–O andcontaining one (or preferably more) crystal planes per unit cell that are consisting of onlyCu and O atoms in a planar square lattice, cf. Table 1. In order to keep electroneutralitythe MeO must be strictly 2+, which determine the choice of M-cation. The cations withthe formal charge 4+ exhibit only small ionic radius and thus can bear an insufficientnumber of neighboring oxygens. The solution grant access to structures either with defi-ciency of anions (YBa2Cu3Ox) or exhibiting more combine constitution (Tl, Hg-basedstructures). The basic configuration reveals three parallel layers, [CuO2][MO][M’O],necessarily associated with overlapping of electron orbitals (Cu – 3dx2–z2 and O – 2px,z).As mentioned, superconductivity originates from the strongly interacting electrons inthese CuO2 planes (Cu atoms are believed to be in the Cu2+ – 3d 9 configuration with thespin ½ so that the overall electronic state is the antiferromagnetic Mott insulator). Dopingthese insulating CuO2 layers with holes (or electrons) causes the appearance of new elec-tronically ordered state which includes that of superconductivity because the mutual hop-ping of electrons from Cu to Cu becomes possible. A generally known schematic phase

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���������� ���: HIGH-Tc OXIDE SUPERCONDUCTORS 369

diagram of such cuprates reveals ‘optimum’ hole-doping near the hole concentrationabout 0.2 per Cu atom (with inter-atomic distance of about 0.3 nm, the Fermi wavelengthof about 1 nm and superconducting coherence length of about 1.5 nm).

Important role plays the complex ‘ceramic’ morphology of these materials.Most HTSC are composed of grains (expectantly to be oriented along the current) theinterface of which is responsible for the composition adjustments by enabling desir-able easy-interaction with surrounding oxygen. Therefore we can expect intimate re-lation between HTSC properties and the material morphology depending on the wayof foregoing technological treatment. We try to avoid the formation of insulating lay-ers, the contamination by reactive gases (CO2, H2O) or the incorporation of impuri-ties from the sample holders or precursors, although, some impurities may turn outagreeable when serving as pinning centers (such as unreacted Y2O3 itself) forsuper-current vortexes.

Although we use to take care about off-stoichiometry and second phases occur-rence in bulk we have to be also careful about the composition peculiarities on the in-terfaces where high curvature of the YBa2Cu3Ox crystallites may even change the lo-cal oxygen stoichiometry, x, by depletion of oxygen on surface stress sites with theconsequent formation of week links, Fig. 2a. Tentatively, it may result in the drift ofthe low-temperature curve tails experimentally measured on the plot of relative con-ductivity vs. temperature. Similar consequence was early studied by the electric fieldeffect resulting in the field-driven oxygen rearrangement [30] arising from thefield-driven movement of the mobile charge carriers in HTSC being thus a genericproperty of these superconductors.

Another awkward effect can be found in the HTSC vicinity layer of super-current channels where cation stoichiometry may not stay stable but its intimate links

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370 ���������� ���: HIGH-Tc OXIDE SUPERCONDUCTORS

Fig. 2 a – Schematic plot of relative electric resistivity, Rr, vs. temperature, T, showingthe effect of higher surface energy on stress sites along the highly-curved inter-faces. It may induce rather negligible changes in the surface oxygenstoichiometry but, nevertheless, it may remain effective enough to modify thecourse of the tailing curve descending after the inflection point to minimum re-sistivity at low temperatures. b – Schematic plot of thermal dependence of mag-netic susceptibility, χ, at low temperatures, T, may well testify its stepwisechange as the curve exhibits the ascension of superconductivity due to the grad-ual inter-linking of individual structural blocks at different levels of their effi-cacy at flat (chains, areas) and space (sector) compass

can be strongly modified [21]. The real (111) composition, with the oxygen content atx = 6.85, can contain both the two- and trivalent cations of copper as well as holes (�)such as YBa2Cu2.6Cu0.4�0.15O6.85. Under the effect of negative-charge ofsuper-current flow on the very thin neighboring layers it may induce not only thephase structure deformation but also certain chemical changes due to the possible ab-sorption of some species, likely Ba2+ and�2+ , that can migrate from the internal bodyof HTSC. The intact interface composition can then carry through a curiously widerange of stoichiometry, e.g.,

YBa3.03–3.09Cu0.392–0.26Cu0.4�0.223–0.226O5.744–5.677

as theoretically shown in [21].Yet another source of curiosity may be established when making SQUID mea-

surements on the YBa2Cu3Ox single crystals [31] where we can witness stepwise se-quences of gradual transfer of dimensionality, D, from the 2-D to 3-D superconduct-ing state, Fig. 2b. Although first indication of superconductivity turns up to be as highas 92 K the internal dissipation processes hold back its instantaneous progress downto 85 K. It is because the [CuO2]

2– layers turn more easily to the superconducting stateon cooling (already at T1=91.2 K) while at 2 K lower the same is done by wholeblocks of ([CuO2]

2– [Y]3+ [CuO]2) until the whole-bulk superconductivity is achievedat yet 4 K lower via tunneling Ba–CuO–Ba barriers. It, however, does not complywith the classical BCS theory but it seems to be more correlative to the antiferro-magnetic arrangement in 2-dimensional systems.

Thermodynamics and BCS theory

Onset of superconductivity bears special features because all linear dc transport coef-ficients disappear. Material parameters are thus deduced rather from equilibriumproperties, would they be of the thermodynamic nature or from the spectral measure-ments. Apparently the HTCS escape the traditional BCS [27] theory including itsgeneralization by Eliashberg to strongly coupled superconductors [29]. The criticaltemperature is nearly by an order of magnitude larger than it is common for the con-ventional superconductors of a comparable electronic density of states. Moreover, thecritical values of temperature, Tc, magnetic field, Hc, and magnetostriction, dV/dH,the thermal capacity, cp, the discontinuity of cp at the critical point, and the energy gapare not interrelated by ratios found in conventional superconductors. This fact imme-diately boosted a search for a new mechanism of the superconductivity.

While in the study of conventional superconductors the major headway wasmade possible by physical measurements (discovery of the superconductivity itself,the Meissner effect, the isotop effect, the energy gap seen first in the specific heat andlater in the tunneling, and so on), in the case of HTSC the most important test of newtheories provided a large variety of materials in which the CuO2 planes are embeddedin quite different ionic backgrounds. Chain-free materials soon ruled out the very firsttheories based on the superconductivity of CuO chains, only. Similarly, materials inwhich the assumed ingredient is missing eliminated the ionization of the apical oxy-

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gen, the orthorhombic distortion, the breezing phonon mode, and other couplingmechanisms. For theories based on the dominant role of CuO2 planes the crucial testwas the sensitivity of the critical temperature to the substitution of electronicallyneighboring zinc into Cu(2) positions.

The physical measurements turned in a great part towards magnetic properties ofthe HTSC. In the conventional isotropic superconductors the magnetic field is either ex-pelled or is forming the Abrikosov vortex lattice. The THSC materials are, in general,highly anisotropic with the anisotropy ratio ranging from moderate values leading to theanisotropic three-dimensional behavior to large values showing clear two-dimensionalfeatures. These regimes make possible to achieve new phases of the vortex matter wouldthey be the vortex liquid, braking of vortices into pancakes, or the system mutually or-thogonal Josephson and Abrikosov vortices [29] penetrating each other in two-dimensional-like systems. Though rearrangements of the vortex matter correspond torather small changes of the free energy, these transitions are detectable by the specificheat. Such thermodynamic measurements together with magnetic measurements, neutrondiffraction and direct observation of the structure via vortex decoration provide the ex-perimental background for the vortex matter comparable to the early studies of phasetransitions in solids. The theory has followed this step from back into the history as theinterest has shifted from the microscopic treatment towards the phenomenological ap-proach of Ginzburg–Landau type. Some advanced calculation [32] were done in studyingthe electrostatic potential in a superconductor applying the Maxwell equation for the vec-tor potential, the Schroediger equation for the wave function and the Poisson equation forthe electrostatic potential. It follows that the electrostatic and the thermodynamic poten-tial compensate each other to a great extent resulting into an effective potential acting onthe superconducting condensate.

* * *

The Grant Agencies supported the study, in particular it was GA �R, project No. 401/02/0579(Methods and practice of transdisciplinary understanding) concluded at the Center of TheoreticalStudies of Charles University in Prague, as well as by GA AV �R, project No. A4010101 (Study ofcrystallization and transformation processes). The study was also fulfilled under the cooperationproject MFM No. 230000009 (Research and modeling of systems and processes of multiscale char-acter), concluded at the Institute of Interdisciplinary studies, West Bohemian University in Pilsen.

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