lo straordinario incremento di nuovi minerali della tormalina negli ultimi tre anni

Lo straordinario incrementodi nuovi minerali della tormalina

negli ultimi tre anni

FERDINANDO BOSIDipartimento di Scienze della Terra, Sapienza Università di Roma

Tourmalines are borosilicates represented by the general formula:XY3Z6(T6O18)(BO3)3V3W

[9]X = Na, K, Ca, vacancy;[6]Y = Al, Cr, V, Fe, Mg, Mn, Li etc. ;[6]Z = Al, Cr, V, Fe, Mg;[4]T = Si, Al, B;[3]B = B;[3]W(O1) = OH, F, O;[3]V(O3) = OH, O.

Tourmalines occur in a wide variety ofsedimentary, igneous, and metamorphic rocks.

The best-known species probably are:

DraviteNaMg3Al6(Si6O18)(BO3)3(OH)3(OH)

SchorlNaFe3Al6(Si6O18)(BO3)3(OH)3(OH)

ElbaiteNa(Al1.5Li1.5)Al6(Si6O18)(BO3)3(OH)3(OH)

TOURMALINE IS INTERESTING…

– as a MINERAL– as a GEMSTONE

– as a PETROLOGICAL INDICATOR

– as a MATERIAL for technological applications

as a gemstone

Tourmaline was “discovered” as a gemstone.

In fact, the term tourmaline seems to be derived from the Sinhalese word turmali, which was used to refer to mixed-

colored stones of unknown type by gem dealers in Ceylon (now Sri Lanka).

“MOTHER NATURE’S RAINBOW” Gem tourmaline is famous for its extensive range of colors, even

within individual crystals: from colorless, through red, pink, yellow, orange, green, blue, and violet, to brown and black.

as a gemstone

Tourmaline gem varieties are often known on color basis

Rubellite (rose, dark pink, to red)

as a gemstone

Verdelite (green to yellow-green)

as a gemstone

Indicolite (blue to blue-green)

as a gemstone

Achroite (colorless)

as a gemstone

Canary tourmaline (yellow)

as a gemstone

Chrome tourmaline (vivid green)

as a gemstone

Paraíba-type(“neon” blue-to-green)

is one of the highest-priced colored gemstones (values comparable to those of some diamonds, Pezzotta and Laurs 2011)

as a gemstone

Cat’s eye and moor’s head tourmalines

as a gemstone

As tourmalines are sensitive to physicochemicalchanges in their growth environment, they may be optically zoned.

as a gemstone

Cut stones are often mounted into jewelry

Pendant consisting of two Cu-bearing tourmalines (10.95 ct pink and 6.95 ct yellow) set in 18 k gold with diamonds

as a gemstone

TOURMALINE STRUCTUREis one of the most complex as well as the most elegant of all

crystal structures of rock-forming minerals

as a mineral

The cyclosilicate structure is formed by rings of six TO4 tetrahedra, which point in the same direction.

Thus, the structure results both noncentrosymmetric and polar: thus, tourmaline is both piezoelectric and pyroelectric

The tourmaline structureXY3Z6(T6O18)(BO3)3V3W

as a mineral

T-site


Tourmaline supergroup can be classified into primary groups based on the dominant occupancy of the X site:

vacant, alkali and calcic groups.This grouping makes sense because X-site occupancy usually

reflects the paragenesis of the rock in which these tourmalines crystallize

as a mineral

X-site

The most extensive compositional variation occurs at the Y site.


Y-site is able to incorporate cations of different sizes and charges, including vacancies.

as a mineral

Y-site


Boron makes tourmaline one of the most important boron-bearing minerals (reservoir of B) in the Earth’s crust.

as a mineral

B-site

Structural islands


as a mineral

“X+Y+B+T”

ZO6 octahedra link the structural islands


The 3-D framework is given by ZO6

as a mineral

Z-site

The tourmaline structureProjected onto (0001)

as a mineral

The tourmaline structure

The 3-D framework of ZO6 explains the tourmaline hardness (7-7½ Mohs) and lack of cleavage, making tourmaline a

resistant mineral in clastic sediments.

as a mineral

Tourmaline structure can accommodate a large range of chemically different elements:

XY3Z6(T6O18)(BO3)3V3W[9]X = Na, Ca, Vac. >> K, Pb, Ag[6]Y = Al, Cr, V, Fe3+, Fe2+, Mg, Mn3+, Mn2+, Li >> Ti, Zn, Cu, Ni, Co, Vac., etc.[6]Z = Al, Cr, V, Fe3+ > Mg, Fe2+

[4]T = Si >> Al, B, Be[3]B = B[3]W(O1) = OH, F, O[3]V(O3) = OH, O

BUT ITS CRYSTAL CHEMISTRY IS CONTROLLED BYSTRUCTURAL CONSTRAINTS

as a mineral

The 3-D framework of ZO6 must be able to accommodate the structural islands

as a mineral

Spatial relationships and reciprocal constraints of ZO6 and YO6:the islands made of 3 Y are surrounded by continuous Z skeleton

as a mineral

As YO6 is larger than ZO6, there is mismatch between these two non-equivalent distorted octahedra

Structural constraints on chemical variability

as a mineral

So far…

as a mineral

Long-range constraints

254 data from SREF

LONG-RANGE DIMENSIONAL CONSTRAINTS

Order-disorder reactionYAl3+ + ZMg2+ → YMg2+ + ZAl3+

applies to the tourmaline to reduce the misfit between

<Y-O> and <Z-O>.

By the incorporation of smaller cations (R3+) into Y

and larger cations (R2+) into Z,

<Y-O> decreases and <Z-O> increases

as a mineral

TOURMALINE CLASSIFICATION

The general formula:

XY3Z6(T6O18)(BO3)3V3W

[9]X = Na, K, Ca, vacancy;[6]Y = Al, Cr, V, Fe, Mg, Mn, Li etc. ;[6]Z = Al, Cr, V, Fe, Mg;[4]T = Si, Al, B;[3]B = B;[3]W(O1) = OH, F, O;[3]V(O3) = OH, O.

The dominance of these ions at one or more sites of the structure gives rise to a range of distinct mineral species

Tourmaline is, in fact, not a single mineral but a supergroup currently consisting in 27 species approved by IMA-CNMNC

IMA-ACCEPTED TOURMALINE SPECIESFrom Henry et al. (2011)1 – Dravite 2 – Schorl3 – Elbaite4 – Fluor-dravite5 – Fluor-schorl6 – Povondraite 7 – Rossmanite8 – Fluor-buergerite9 – Olenite 10 – Uvite11 – Fluor-uvite12 – Feruvite 13 – Fluor-liddicoatite 14 – Foitite15 – Magnesio-foitite 16 – Chromo-alumino-povondraite17 – Chromium-dravite

18 – Oxy-schorl (Bačik et al., IMA 2011-011)19 – Tsillaisite (Bosi et al., IMA 2011-047)20 – Fluor-elbaite (Bosi et al., IMA 2011-071)21 – Oxy-chromium-dravite (Bosi et al., IMA 2011-097)22 – Oxy-vanadium-dravite (Bosi et al., IMA 2012 11-E)23 – Oxy-dravite (Bosi et al., IMA 2012-004a)24 – Darrellhenryite (Novák et al., IMA 2012-026)25 – Vandio-oxy-chromium-dravite (Bosi et al., IMA 2012-034)26 – Fluor-tsilaisite (Bosi et al., IMA 2012-044)27 – Vanadio-oxy-dravite (Bosi et al., IMA 2012-074)

The last 3 years have seen an amazing increase in tourmaline species: 17 + 10 = 27

Dravite Na Y(Mg3) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(OH)

Fluor-dravite Na Y(Mg3) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(F)

Oxy-dravite Na Y(MgAl2) Z(MgAl5) (Si6O18) (BO3)3 (OH)3 W(O)

Schorl Na Y(Fe3) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(OH)

Fluor-schorl Na Y(Fe3) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(F)

Oxy-schorl Na Y(Fe2Al) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(O)

Elbaite Na Y(Li1.5Al1.5) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(OH)

Fluor-elbaite Na Y(Li1.5Al1.5) Z(Al6) (Si6O18) (BO3)3 (OH)3 W(F)

Darrellhenryite Na Y(LiAl2) Z(Al6) (Si6O18)(BO3)3(OH)3W(O)

Tsilaisite Na Y(Mn3) Z(Al6) (Si6O18)(BO3)3(OH)3W(OH)

Fluor-tsilaisite Na Y(Mn3) Z(Al6) (Si6O18)(BO3)3(OH)3W(F)

Rossmanite � Y(LiAl2) Z(Al6) (Si6O18)(BO3)3(OH)3W(OH)

Foitite � Y(Fe2Al) Z(Al6) (Si6O18)(BO3)3(OH)3W(OH)

Magnesio-foitite � Y(Mg2Al) Z(Al6) (Si6O18)(BO3)3(OH)3W(OH)

Flour-liddicoatite Ca Y(Li2Al) Z(Al6) (Si6O18)(BO3)3(OH)3W(F)

Olenite Na Y(Al3) Z(Al6) (Si6O18)(BO3)3(O)3W(OH)

Fluor-buergerite Na Y(Fe3+3) Z(Al6) (Si6O18)(BO3)3(O)3

W(F)Feruvite Ca Y(Fe2+

3) Z(MgAl5) (Si6O18)(BO3)3(OH)3W(OH)

Uvite Ca Y(Mg3) Z(MgAl5) (Si6O18)(BO3)3(OH)3W(OH)

Fluor-uvite Ca Y(Mg3) Z(MgAl5) (Si6O18)(BO3)3(OH)3W(F)

Chromium-dravite Na Y(Mg3) Z(Cr6) (Si6O18)(BO3)3(OH)3W(OH)

Povondraite Na Y(Fe3) Z(Mg2Fe4) (Si6O18)(BO3)3(OH)3W(O)

Chromo-alumino-povondraite Na Y(Cr3) Z(Mg2Al4) (Si6O18)(BO3)3(OH)3W(O)

Oxy-chromium-dravite Na Y(Cr3) Z(Mg2Cr4) (Si6O18)(BO3)3(OH)3W(O)

Oxy-vanadium-dravite Na Y(V3) Z(Mg2V4) (Si6O18)(BO3)3(OH)3W(O)

Vanadio-oxy-chromium-dravite Na Y(V3) Z(Mg2Cr4) (Si6O18)(BO3)3(OH)3W(O)

Vanadio-oxy-dravite Na Y(V3) Z(Mg2Al4) (Si6O18)(BO3)3(OH)3W(O)

Nomenclature

Tourmaline classification (Henry et al. 2011)

Tourmaline classification of Henry et al. (2011, 2013)

XY3Z6(T6O18)(BO3)3V3W

“For the purposes of classification of tourmaline species,actual tourmaline structural information of the Y- and Z-site occupancy is an overriding consideration for the definition of a tourmaline species”

Henry et al. (2013).

Empirical (real) structural formula has to be usedin naming the tourmaline

Hence, accurate site allocation of cations and anions is needed !

Empirical structural formula of Clark et al. (2011): XNaY(Mg2+

1.4Al3+0.6Fe2+)Z(Al3+

5.4Mg2+0.6)T(Si6O18)B(BO3)3

V(OH)3W[F0.7(OH)0.3]

Fluor-dravite, end-member formula NaY(Mg3)Z(Al6)(Si6O18)(BO3)3(OH)3(F)

x < 0.2 (for example, x = 0.1)Y(Fe2+

1.4Mg1.5Al0.1) Z(Mg0.1Al5.9)

Structural formula:Na Y(Fe2+

1.4Mg1.6-xAlx) Z(MgxAl6-x) (Si6O18)(BO3)3(OH)3F



x > 0.2 (for example, x = 0.3)Y(Fe2+

1.4Mg1.3Al0.3) Z(Mg0.3Al5.7)

Nomenclature

x < 0.2, fluor-dravitex > 0.2, fluor-schorl

For the same bulk chemistry, the name changes as a function of the degree of order/disorder over Y and Z



Oxy-vanadium-draviteX(Na)Y(V)3

Z(V4Mg2)T(Si6O18)(BO3)3V(OH)3

W(O)

Oxy-chromium-draviteX(Na)Y(Cr)3

Z(Cr4Mg2)T(Si6O18)(BO3)3V(OH)3

W(O)

Vanadio-oxy-chromium-draviteX(Na)Y(V)3

Z(Cr4Mg2)T(Si6O18)(BO3)3V(OH)3

W(O)

OXY-TOURMALINES

Vanadio-oxy-draviteX(Na)Y(V)3

Z(Al4Mg2)T(Si6O18)(BO3)3V(OH)3

W(O)

Oxy-draviteX(Na)Y(Al)3


W(O)

Chromo-alumino-povondraite X(Na)Y(Cr)3


W(O)

Oxy-chromium-draviteOxy-vanadium-dravite

Vanadio-oxy-chromium-draviteVanadio-oxy-dravite

Chromo-alumino-povondraite

OXY-TOURMALINES

The 5 new species of Cr-V-oxy-tourmalinesoccur in the Pereval marble quarry, near the town of Sludyanka

(51°37′N 103°38′E), Irkutsk region, Southern Lake Baikal, Siberia, Russia

The Sludyanka complex comprise sedimentary-metamorphic rocks consisting of diverse gneisses, carbonate, and carbonate-silicate

rocks and mafic schists

Probably, Cr-V-oxy-tourmalines were formed in the prograde stage of metamorphism (i.e., granulite facies)

Ternary diagram for V-Cr-Al oxy-tourmaline NaYR3+3

Z(R3+4Mg2)

(Si6O18)(BO3)3(OH)3O

“Intermediate" end-members

What are their compositional fields in the diagram V-Cr-Al?

Crtot = 5.0 and Altot = 2.0



Example, chromo-alumino-povondraite:it is between oxy-chromium-dravite and oxy-dravite join.

ZCr2↔ZAl2

YCr1.5↔YAl1.5


Z(R3+4Mg2)



Z(R3+4Mg2)


Site preference:YV > YCr > YAlZAl > ZCr > ZV

?


Z(R3+4Mg2)


Site preference:YV > YCr > YAlZAl > ZCr > ZV

Confirmed by

a systematic study

(work in progress)

Nomenclature

(Henry et al. 2013)“In the absence of specific structural information on the Y- and Z-site occupancies, a procedure is recommended for allocating cations to Z and Y ... Initially assign all Al3+ (in excess of that assigned to T) to the Z site. Next, successively assign Mg2+ (up to 2

apfu), V3+, Cr3+, and Fe3+. If there is an excess of trivalent cations on Z, it goes into Y”.

Chemical analysis, sample PR1973(Na ~1 apfu, Mg ~2, V3+ ~2.2, Cr ~3.8 , Al ~1, Si ~6, B = 3, OH ~3)

Recommended formula according to Henry et al. (2013): NaY(Cr3)Z(Al1Mg2V2.2Cr0.8)(Si6O18)(BO3)3(OH)3

W(O)

Empirical formula according to the structural information: NaY(Cr0.8V2.2)Z(Al1Mg2Cr3)(Si6O18)(BO3)3(OH)3

W(O)

Nomenclature

(Henry et al. 2013)“In the absence of specific structural information on the Y- and Z-site occupancies, a procedure is recommended for allocating cations to Z and Y ... Initially assign all Al3+ (in excess of that assigned to T) to the Z site. Next, successively assign Mg2+ (up to 2

apfu), V3+, Cr3+, and Fe3+. If there is an excess of trivalent cations on Z, it goes into Y”.

Chemical analysis, sample PR1973(Na ~1 apfu, Mg ~2, V3+ ~2.2, Cr ~3.8 , Al ~1, Si ~6, B = 3, OH ~3)

Recommended formula according to Henry et al. (2013): NaY(Cr3)Z(Al1Mg2V2.2Cr0.8)(Si6O18)(BO3)3(OH)3

W(O)

Empirical formula according to the structural information: NaY(Cr0.8V2.2)Z(Al1Mg2Cr3)(Si6O18)(BO3)3(OH)3

W(O)

Vanadio-oxy-chromium-draviteX(Na)Y(V)3

Z(Mg2Cr4)T(Si6O18)(BO3)3V(OH)3

W(O)

Ternary diagram for V-Cr-Al oxy-tourmaline NaYR3+3Z(R3+

4Mg2)


Boundaries within the diagram?

Ternary diagram for V-Cr-Al oxy-tourmaline in accord with the recommendations of Henry et al. (2013), except for V3+ and Cr3+

1) Y(Al1Cr1V1) Z(Al4Mg2)

2) Y(Cr1.5V1.5) Z(Al4Mg2)

3) Y(Cr1.5V1.5) Z(Cr2Al2Mg2)

4) Y(V3) Z(Cr2Al2Mg2)

5) Y(V3) Z(V1.33Cr1.33Al1.33Mg2)

NaY(Cr1.4V1.6)Z(Mg2Cr1.9Al2.1)(Si6O18)(BO3)3(OH)3OCrtot = 3.3 > Altot = 2.1 > Vtot = 1.6

Vanadio-oxy-dravite


Coming soon…Special Collection on Spinels in American Mineralogist

Spinels Renaissance:The past, present and future of those ubiquitous materials

A special collection, focused on diverse topics, related to the structure, properties and applications of natural and synthetic spinels and spinelloids on bulk and nanoscale. The section aims at the revival of the interest in the spinel materials at present and particularly on the promising future of the non-oxygen containing and nanosized structures. We hope to bring together experimental and theoretical research studies from mineralogists, geologists, chemists, materials scientists, physicists and crystallographers.

Papers will undergo normal peer review, conducted by special collection associate editors Kristina Lilova, Kaimin Shih and Ferdinando Bosi.

lo straordinario incremento di nuovi minerali della tormalina negli ultimi tre anni

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