chemical characteristics of granites with a · in the natural stone industry the term granite has a...

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

ISSN 1400-3821 B631 Bachelor of Science thesis Göteborg 2011

Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

Chemical characteristics of granites with a

discolouration potential

Joachim Andersson

1

Abstract

The chemistry of six leucogranites with a clear discolouration was studied using SEM-EDS

and the polarizing microscope. The Bohus granite was used as a reference, because it is

considered as a durable natural stone. All samples are currently sold as natural stones. Three

of them are slightly weathered before use. The granites are fractionated I-type granites, which

mean that they have a magmatic source and evolved through magmatic processes. They have

low contents of Ti, Mn, Mg, Ca, high field strength elements and rare earth elements

compared to most granites. It is uncertain how or if the fractionated nature of the granites

affects their overall resistance to weathering. Iron leakage from biotite is most likely the cause

of discolouration in all samples, although the cause of discolouration in Granite 2 remains

more uncertain because it is the only fresh sample. Weathering products high in Mn were

found in Granite 3 and are probably caused by biotite which has a relatively high Mn content

in relation to the bulk concentration of Mn. The Mn content in biotite is 60% higher (1.67

wt.% MnO) compared to the other samples. Mn is generally concentrated in a few minerals

and could explain a part of the discolouration. High density of large cracks in large quartz

grains is probably the major cause of high weathering rates and rapid propagation of the

discolouration.

Sammanfattning

Kemin hos sex leucograniter med tydlig missfärgning har analyserats med SEM-EDS och

polarisationsmikroskopering. Bohusgraniten har används som referens, eftersom den anses

vara av god kvalitet. Alla prover säljs för tillfället som natursten. Tre av dem är något vittrade

innan användning. Graniterna är fraktionerade I-graniter, vilket innebär att de har en

magmatisk källa och utvecklats genom magmatiska processer. De har låga halter av Ti, Mn,

Mg, Ca, high field strength elements och sällsynta jordartsmetaller jämfört med de flesta

graniter. Det är oklart hur eller om detta påverkar provernas generella vittringsbenägenhet.

Järn läckage från biotit är den mest troliga orsaken till missfärgningarna i samtliga prover,

men orsaken till missfärgning i Granite 2 är mer osäker eftersom provet är det enda ovittrade

som analyserats. Vittringsprodukter med hög Mn-halt hittades i Granite 3 och beror troligtvis

på biotit, vilken är Mn-rik i förhållande till bulkkoncentrationen av Mn. Mn-koncentrationen i

biotit är 60% högre (1.67 viktprocent MnO) än i övriga prover. Mn är koncentrerat i få

mineral och skulle kunna förklara en del av missfärgningen. Hög koncentration av stora

sprickor i stora kvartskorn är troligen den dominerande orsaken till de höga

vittringshastigheterna och missfärgningen.

Keywords: granite; natural stone; weathering; discolouration; biotite

2

Contents

Abstract ..................................................................................................................................... 1

Sammanfattning ....................................................................................................................... 1

Contents ..................................................................................................................................... 2

Introduction .............................................................................................................................. 3

Commercial granites ............................................................................................................................ 3

Granite weathering .............................................................................................................................. 4

Methods ..................................................................................................................................... 6

Results ....................................................................................................................................... 7

Whole-rock chemistry ......................................................................................................................... 7

Mineral compositions ........................................................................................................................ 13

Petrography ....................................................................................................................................... 13

Discussion ................................................................................................................................ 19

Conclusions ............................................................................................................................. 20

Acknowledgements ................................................................................................................. 21

References ............................................................................................................................... 21

Appendix ................................................................................................................................. 23

Contents

3

Introduction

The aim of this study is to investigate six leucogranites (light-colored granites), that have

become discoloured when used as natural stone, and try to find the causes for their high

weathering and discolouration rate. If common characteristics that could cause the rapid

discolouration are found, a test procedure or recommendation can be worked out in order to

prevent such granites from entering the Swedish market or being used in applications that can

trigger discolouration. The granites will be classified using the S-I-A-M classification for

granitoid rocks (Winter 2001) in order to find similarities or differences in their paragenisis.

The physical and micro-structural characteristics of these rocks such as cracks and grain

boundaries are not primarily a part of this work, but will be paid some attention. This study is

a joint project in the collaboration between Earth Sciences Centre, Göteborg University

(GVC) and the Swedish Cement and Concrete Research Institute (CBI), Borås. Samples were

chosen by CBI and the additional Bohus granite was donated by Fredrik Henriksson,

Henrikssons stenhuggeri.

All six granites are currently used as natural stones, and all have become discoloured

after only few months in their application/environments respectively (Fig. 1.). Three of the

granites were yellow even before use (Granite 1, 4 and 5). Low-cost, yellow granites from

China have become fairly common in Sweden recently, obviously due to the price but also

due to their unusual color which appeals to architects among others. As geologists know,

yellow rocks are rarely of good technical quality for to natural stone usage. The origin of the

studied rocks is, in cases, poorly known. Sample Granite 4 is from Leizhou, southeastern

China and is the only located rock. The other five samples are either from China, Italy or

Portugal. Unfortunately there are not both before and after weathering samples from the same

rock. All samples are from different rocks and can be grouped into three categories; one fresh

unused rock, two weathered used rocks and three unused yellow rocks. Because of this

inconvenience the samples will not be considered as six separate problems, instead there will

be more of a “put the puzzle together” approach to this study. The Bohus granite is considered

a high quality natural stone and will therefore be used for comparisons. The samples and the

Bohus granite are shown in (Fig. 2.)

Commercial granites

In the natural stone industry the term granite has a much wider meaning and can

roughly be defined as a hard polishable natural stone significantly harder than marble. 64% of

the world’s commercial granites are extracted in three countries: India (32%), Brazil (22%)

and USA (10%). Precambrian rocks are overrepresented. Scandinavia supplies 10.5% of the

world’s commercial granites (Sweden supplies 1%) and most of them are Precambrian. Italy,

China and Portugal provide 4%, 3.5% and less than1% respectively of the world’s

commercial granites. For those countries phanerozoic rocks dominate. Commercial granites

from Italy and Portugal are primarily Carboniferous in age and are associated with the

Hercynian orogeny (~300 Ma). The geological positions of Chinese granites are not as well

known to European buyers as European granites. There are several Cretaceous granites from

the Fujian province in southeastern China (Pivko 2004). Fujian is a neighbor province to

Guangdong, where Leizhou is situated. Two of the non-yellow granites included in this study

are very similar to granites from this region, but most pictures are of low quality and rocks

can have up to 50 synonym names. Given the recent problems regarding yellow granites from

China, the possibility exists that all samples are of Chinese origin, but there is a great

4

uncertainty. The problems with discoloured granites from China and southern Europe are

discussed by Schouenborg et al. (2010).

Granite weathering

The weathering rate of granites depends on many variables. Oxidation is often the first form

of weathering to affect a rock, removing iron from ferromagnesian minerals and destroying

the crystal structure. The weathering of weak minerals such as biotite often leads to

decomposition of the entire rock due to the accompanying volume increase. When Fe2+

- ions

in the biotite becomes exposed to oxygen and water they oxidize to Fe3+

which have a low

solubility and therefore precipitate to form solid iron-hydroxides which increases the volume

of the rock. The mobility for common ions in order from least to most mobile are: Ca2+

, Na+,

Mg2+

, K+, Fe

2+, Si

4+, Ti

4+, Fe

3+ and Al

3+. Coarse-grained rocks usually weather faster than fine

grained, even though the latter have larger total grain surface (Easterbrook 1993).

The weathering susceptibility of rock-forming minerals is roughly related to magmatic

crystallization order. Olivine and anorthite are for example more easily weathered than K-

feldspar and quartz, but micro-features and external factors such as organic acids can greatly

affect the resistance as well. Higher density of dislocations, defects and exsolution features

increases the weathering rate. Points of weakness are preferentially weathered and therefore

of great significance. If a K-feldspar is perthitic the mineral will weather faster because the

albite lamellae are preferentially attacked (Wilson 2004).

Discolouration of granites is usually linked to iron-bearing minerals such as biotite,

hornblende and pyrite, but can also be caused by precipitation of limonite in cracks due to

ferruginous surface water (Dale 1907). The weathering of biotite is complex and can result in

a number of different secondary minerals such as vermiculite, gibbsite, Al-chlorite, smectite,

kaolinite and sagenite (Bisdom et al. 1982). The most characteristic reaction is called

vermiculitization which involves exchange of the interlayer K for external ions. Natural

vermiculitization sometimes involves a process in which a hydrobiotite, biotite-vermiculite, is

formed. The reaction also includes oxidation of octahedral iron (Wilson 2004).

Figure 1. Discolouration of two Chinese granites. To the left, one in Stockholm and to the right, one in

Copenhagen.

5

Fig. 2. Rocks used in this study: a) Bohus granite, b) Granite 1, c) Granite 2, d) Granite 3, e) Granite 4, f) Granite

5 and g) Granite 6.

6

Methods

Thin sections for samples Granite 2-4 and the Bohus granite where prepared at the Earth

Sciences Centre, University of Gothenburg. Thin sections for samples Granite 1 and Granite 6

were prepared by Teknologisk Institut, Taastrup, Denmark. Two additional thin sections from

Granite 3, impregnated with yellow fluorescent epoxy, were borrowed from CBI. Whole-rock

samples were crushed and ground at CBI, Borås (Fig. 3.). Whole-rock chemical analyses,

including determination of the ferrous iron content, were carried out by Service d'Analyse des

Roches et des Mineraux, Vandoeuvre, France. Mineral analyses in polished thin sections and

backscattered electron (BSE) images were performed using an energy dispersive spectrometer

(EDS) attached to a scanning electron microscope (SEM) at the Earth Sciences Centre,

Göteborg University. Optical studies were done using transmission and reflection microscopy.

Whole-rock data were plotted using plotting programs GCDkit 2.3 and GeoPlot.

Fig. 3. Crushing (left) and grinding (right) at CBI, Borås.

7

Results

When looking at hand samples one can see that the discolouration is almost exclusively

associated to biotites (Fig. 2.). In thin section these discolouration is seen as cracks filled by

yellow to brown amorphous mush (Fig. 4.). This mush is branching out of biotites in most

cases. It looks like the biotites are leaking. Some of these mushes (possible leakages) were

analysed with SEM-EDS and are presented in Table 6 in appendix.

Fig. 4. Biotite and opaques with yellow brown mush branching out. Picture from Granite 1 in plane polarized

light.

Whole-rock chemistry

The analytical data for major and minor elements are presented in Table 1. The whole rock

data were plotted in geochemical diagrams, commonly used in granite petrology, in order to

shed some light on the geologic history of the rocks, which could be of importance. All

granites are rich in Na2O and have low concentrations of TiO2, Fe2O3, FeO, MnO, MgO, CaO,

Al2O3, K2O and P2O5 compared to the Bohus granite, but the differences are relatively small.

All samples have similar major and minor element compositions and plot as granites or alkali

granites in the R1-R2 plot (Fig. 5.). Granite 2 and Granite 5 have a slightly more mafic

8

signature. All samples are slightly peralumnious (Fig. 6.) and have normative corundum

values between 0.3 and 0.8 wt.%. Trace element compositions are much more variable. The

spider diagram rock/mid ocean ridge basalt (Fig. 7.) shows a high large ion lithophile

elements/high field strength elements ratio typical of tectonic granites (Whalen et al. 1987).

All samples show a characteristic positive Pb anomaly and negative anomalies for Ba, Nb, P,

Eu and Ti in the primitive mantle spider diagram (Fig. 8.), while Sr values fall into two

groups. Granite 1 and Granite 4 show many similarities; they have the highest Sr and Ba

contents and lowest concentrations of Cs, Nb, Ta, Th, U, Y, Sn and rare earth elements

(except Eu). Granite 5 has the highest content of high field strength elements. The content of

high field strength and rare earth elements are considerably lower for all samples compared to

analyses from the Bohus granite (Pettersson and Eliasson 1997). Rare earth elements in the

Bohus granite are generally 5-10 times higher. Rare earth elements are plotted against

chondrite values in (Fig. 9.). All heavy rare earth element values for Granite 6 look very

unusual and could be due to an analytical error or indicate hydrothermal alteration. All

samples show a negative Eu anomaly, but these are very slight for Granite 1 and Granite 4.

The rare earth element profiles are similar to the profile of the Bohus granite (Pettersson and

Eliasson 1997).

Fig. 5. R1-R2 plot for plutonic rocks (De la Roche et al. 1980).

9

Fig. 6. Al-saturation plot. Molar proportion of Al2O3/(Na2O+K2O) versus Al2O3/(CaO+Na2O+K2O) (Maniar and

Piccoli 1989).

Fig. 7. Sample/Mid Ocean Ridge Basalt. Increasing incompatibility from Yb to Ba and Sr to Ba. MORB values

from (Pearce 1983).

10

Fig. 8. Primitive-mantle normalised spider diagram. Incompatibility decreases from left to right. Primitive

mantle values from (Sun and McDonough 1989).

Fig. 9. Chondrite-normalised REE-profile. Chondrite values from (Boynton 1984).

11

Table 1 Chemical compositions

Sample no. Granite 1 Granite 2 Granite 3 Granite 4 Granite 5 Granite 6

Rock condition Weathered Unweathered Weathered Weathered Weathered Weathered

SiO2 (wt.%) 74.93 72.36 76.84 74.75 72.51 75.80

TiO2 0.06 0.20 0.11 0.06 0.19 0.04

Al2O3 13.91 13.84 12.27 14.35 13.92 12.71

Fe2O3a 0.63 0.58 0.44 0.55 1.10 0.39

FeO 0.19 1.54 0.25 0.27 0.36 0.57

MnO 0.04 0.06 0.04 0.03 0.04 0.05

MgO 0.05 0.44 0.09 0.06 0.25 0.06

CaO 1.12 1.61 0.54 1.14 0.95 0.65

Na2O 4.10 3.19 3.73 4.36 3.97 3.38

K2O 4.26 4.84 4.32 4.43 4.97 4.81

P2O5 < L.D. 0.06 < L.D. < L.D. 0.05 < L.D.

LOI 0.35 0.50 0.27 0.21 0.48 0.40

Total 99.66 99.39 98.91 100.22 98.82 98.92

Cs (ppm) 0.287 2.041 2.003 0.333 1.987 4.197

Rb 98.68 132.5 121.8 129.3 190.9 204.0

Sr 329.1 97.75 79.08 264.9 218.2 26.29

Ba 900.6 563.7 545.6 773.1 657.7 176.1

Be 1.115 1.228 2.926 1.326 4.499 1.557

Nb 3.563 8.686 10.44 3.209 23.26 10.54

Ta 0.162 0.777 1.117 0.158 2.398 1.068

Th 2.766 14.02 9.838 3.687 25.64 19.86

U 0.820 4.120 2.278 0.876 4.167 3.898

Pb 33.33 26.41 26.22 42.78 39.58 35.18

Zr 77.11 129.2 98.45 81.69 208.6 70.54

Hf 2.496 3.906 3.654 2.654 5.976 2.881

Y 8.315 17.06 20.86 8.030 14.87 56.16

V 1.160 18.86 3.211 0.968 9.858 1.579

Co < L.D. 2.156 < L.D. < L.D. 1.295 < L.D.

Cr 4.075 4.385 7.005 5.945 5.210 5.384

Mo < L.D. < L.D. 0.443 0.33 0.428 < L.D.

F 90 350 200 130 550 120

Zn 34.46 48.18 < L.D. 47.15 39.72 24.67

Ga 17.08 16.46 14.65 19.00 20.40 15.98

Ge 1.022 1.500 1.578 1.186 1.436 1.718

Sn 0.737 1.728 1.350 0.697 2.288 3.351

La 6.178 32.64 23.61 6.623 56.60 14.85

Ce 12.32 69.45 50.48 11.53 97.08 32.51

Pr 1.509 7.973 5.660 1.585 9.164 4.117

Nd 5.817 28.64 19.38 5.903 27.55 16.19

Sm 1.363 5.165 3.683 1.415 3.902 4.827

Eu 0.378 0.616 0.500 0.324 0.596 0.239

Gd 1.130 3.637 2.969 1.217 2.556 6.038

Tb 0.171 0.505 0.498 0.176 0.378 1.146

Dy 1.128 2.769 3.027 1.049 2.120 7.970

Ho 0.271 0.519 0.605 0.215 0.402 1.852

Er 0.938 1.508 1.777 0.634 1.215 5.658

Tm 0.168 0.232 0.295 0.102 0.199 0.891

Yb 1.235 1.609 2.045 0.724 1.483 5.975

Lu 0.210 0.243 0.319 0.115 0.248 0.935

Fe3+/∑Fe 0.75 0.25 0.61 0.65 0.73 0.38

A/CNKb 1.04 1.03 1.04 1.02 1.02 1.06

Crnc 0.52 0.57 0.48 0.31 0.40 0.76

Qzd 35.05 36.05 39.53 32.16 30.88 38.86

Abd 37.64 31.05 33.43 39.68 36.88 30.66

Ord 27.31 32.90 27.04 28.16 32.24 30.48

aFe2O3=((Fe2O3

tot×0.8998)-FeO)×1.1113

bAl2O3/(CaO+Na2O+K2O) molar proportion.

c Normative corundum (wt.%)

dNormative quartz, albite or orthoclase (Qz+Ab+Or=100)

12

Ferric iron content is hard to correlate with the grade of weathering, because the ratio can vary

from practically none up to 0.8 in fresh granites (Wilson 1980). In terms of tectonic setting

most granites plot as volcanic arc granites (Fig. 10.), with some overlaps (Pearce et al. 1984).

All granites plot as fractionated granites (Fig. 11.), but Granite 2 plot as unfractionated in the

(K2O+Na2O)/CaO versus Zr+Nb+Ce+Y diagram due to its higher CaO content.

Fig. 10. Geotectonic discrimination diagrams for granitoid rocks. (VAG) volcanic arc, (WPG) within plate,

(ORG) ocean ridge and (syn-COLG) syn-collisional (Pearce et al. 1984).

Fig. 11. Discrimination diagram for granitic rocks. (FG) fractionated S-, I- or M-granites, (OGT) unfractionated

S-, I- or M-granites and (A) anorogenic intraplate granites (Whalen et al. 1987).

13

Mineral compositions

All mineral analyses done with SEM-EDS are presented in the appendix. There are no

analyses of Granite 5 because no thin section was made. The microcline analyses show

normal compositions. Granite 1 has more Ba than the others. Ba containing microclines were

found in all samples but the concentrations did not reach above the detection limit enough

times to be significant and the same apply to Sr, Fe and Ti. The microclines in Granite 3 are

perthitic with an albite composition of An3. Plagioclase compositions vary from albite to

oligoclase (An9-An24). Granite 1, Granite 3 and Granite 4 have very similar plagioclase

compositions (An13-14. All samples have Fe-rich biotites and they are Mn-rich compared to

most biotites in the Bohus granite (Eliasson et al. 2003), but the biotites in the Bohus granite

are more variable (0.05-1.32 wt.% MnO). Biotites in Granite 3 have high Mg and Mn

concentrations given the low whole-rock content and the biotites in Granite 1 have high Ti

concentrations in relation to bulk chemistry. Zn was detected in three of the samples but only

typical of Granite 1. Chloritised biotites are only found in Granite 2 and Granite 6. Altered

biotites in Granite 3 and 4 have lost two thirds of their interlayer K in exchange for water as

seen by the lower K levels and totals. Several different Fe- and Mn-bearing accessory

minerals are found. Mn-rich almandine is only found in Granite 1, Granite 4 and Granite 6.

The garnets in Granite 1 and Granite 4 are almost identical in composition, whereas garnets in

Granite 6 have lower CaO content and no MgO. Large Mn-rich ilmenites (15 wt.% MnO) are

typical of Granite 3. The high Mn-content is possibly due to a solid solution between

pyrophanite (MnTiO3) and ilmenite (Deer et al. 1992). Some ilmenites have exsolution blebs

of more Fe-rich ilmenite (Fig. 12.). Hastingsite amphiboles are typical accessories of Granite

4. A ferric iron estimation was made according to (Leake et al. 1997), in order to make an

amphibole classification. Granite 2 has no Mn- or Fe-bearing accessory minerals except

magnetite. Other accessory minerals confirmed by SEM-EDS include apatite, monazite,

xenotime (YPO4), zircon, epidote, clinozoisite and titanite. No significant zonations were

found.

The composition of mineralised weathering products in cracks is surprisingly diverse.

Some mineralised cracks in Granite 3 have MnO concentrations of up to 40 wt.% and no Fe at

all, while others are of opposite composition with high Fe content and no Mn. The Mn-

bearing minerals adjacent to the mineralised crack with high Mn content, showed no

detectable Mn leakage when analyzed with an element mapping function. The high Al content

of the first and third crack analysis is probably caused by parts of the adjacent minerals being

analyzed too.

Petrography

Granite 1

Grain size: 1-4 mm

Crack width: up to 0.06 mm (both closed and open cracks were measured)

Accessory minerals of significant concentration: Mn-almandine (alm43sp31gro24py2),

hornblende and magnetite.

The cracks in this rock have one distinct orientation with a more diffuse orientation

perpendicular to it. Most long, wide and connected cracks are situated in quartz grains.

Plagioclase has the highest density of cracks, but they are thinner and shorter. Grain

14

boundaries are chaotic and surrounded by quartz clusters. Many feldspars have lost their

euhedral shape due to “invading” quartz. Horblendes are altered. Some microclines are

perthitic and myrmektite is found in some plagioclases. 46% of the opaques are situated far

from biotite, 37% adjacent and17% as inclusions in biotite. The average size of opaques is 0.1

mm and they are round to idiomorphic in shape. There are no convincing examples of Fe-

leaking opaques in this thin section. Heavy iron leakage is most often related to weathered

biotites (Fig. 12.). Many biotites have lost their optical properties.

Fig. 12. Iron leakage from weathered biotite. (a) Backscattered electron image of biotite. Clear quartz grains to

the right and plagioclase to the left. (b) Mapping image showing Fe concentration. Pictures from Granite 6.

Granite 2

Grain size: 3-6 mm

Crack width: up to 0.04 mm

Accessory minerals of significant concentration: epidote, magnetite, monazite, xenotime and

amphibole (unconfirmed).

This rock is the only fresh rock apart from the Bohus granite included in this study. The

sample is pictured together with the Bohus granite in (Fig. 13.) for comparison. The Bohus

granite has a much lower crack density than Granite 2. The cracks have a very distinct

orientation in this rock with a less distinct crack orientation perpendicular to it. Long, wide

and connected cracks in quartz are very typical features in this sample. There are wide

hydrothermally mineralised cracks in the dominant crack orientation which is filled by

sericite. The quartz grains are rather big aggregates and they are connected throughout the

rock. The rocks show many mineralogical similarities with the Bohus granite such as

saussuritised plagioclase (replacement of plagioclase by epidote), chloritised biotite and a

significant amount of Rare earth element mineral inclusions in biotite. Granite 2 has the

second highest concentration of light rare earth elements. Biotites are non- to partly

chloritised and have a lot of inclusions, including magnetite, monazite, Mn-ilmenite, apatite

and zircon. 48% of the opaques are situated adjacent to biotite, 28% far from biotite and 24%

as inclusions in biotite. The average size of opaques is 0.1 mm and they are usually euhedral.

A few microclines are perthitic. No sulphides were found using reflection microscopy.

15

Fig. 13. Representative pictures of polished thin sections taken in plane polarized light. (a) Granite 2 and (b)

Bohus granite. The Bohus granite has a significantly lower density of cracks and defects.

Granite 3

Grain size: 2-4 mm


Accessory minerals of significant concentration: magnetite, muscovite and Mn-ilmenite.

There are one distinct and one diffuse crack orientation in this sample. The cracks are evenly

distributed between feldspars and quartz. The long and wide cracks are found in quartz and

microcline, which are strongly perthitic compared to the other rocks. Many feldspar rims look

remelted (Fig. 14). They have thin rings of small quartz grains surrounding them. Granite 3

has the highest concentration of opaques and magnetite is the dominant phase. Several Mn-

rich ilmenites are found in this rock (Fig. 15.). 43% of the opaques are located adjacent to

biotite, 30% as inclusions in biotite and 27% far from biotite. The average size of opaques is

0.3 mm and they are subeuhedral to euhedral in shape. Biotites vary from amorphous mush to

almost “fresh” in appearance. Iron leakage is clearly linked to biotites, but there are some

examples of possible leakage from opaques. There is a big cluster of sericitised feldspars,

biotites and opaques in this thin section. Two thin sections of this rock, which were borrowed

from CBI, are impregnated with yellow epoxy and clearly show that abundant open small

cracks are associated with the exolution lamellae in microcline.

16

Fig. 14. Backscattered electron image of Mn-ilmenite with exolution pods of more Fe-rich ilmenite. Brightest

minerals are monazite .Picture from Granite 3.

Fig. 15. Picture of perthitic microcline in cross polarized light. Picture from Granite 3.

17

Granite 4

Grain size: 1-3 mm


Accessory minerals of significant concentration: Mn-Almandine (alm43sp30gro25py2),

hastingsite, magnetite and titanite.

There is no clear crack orientation in this sample. The cracks are evenly distributed between

feldspars and quartz. Grain boundaries are rounded and chaotic. Some microclines are

perthitic and a few plagioclases have myrmektite. This sample has many mineralogical

similarities with Granite 1 such as garnet and hornblende, which agrees well with their

similarities in bulk chemistry. Garnet and amphibole crystals are large (>0.3 mm). Most

biotites have lost their shape and are surrounded by heavy iron leakage. 53% of the opaques

are situated adjacent to biotites, 29% far from biotites and 18% as inclusions in biotite.

Opaques are variable in shape from lenticular to round and anhedral to euhedral. The average

size of opaques is 0.1 mm.

Granite 5

Grain size: 1-3 mm

Crack width: unknown

Accessory minerals of significant concentration: magnetite

Unfortunately no thin section was made from this rock. Granite 5 has the highest magnetite

content together with Granite 2 and Granite 3 (checked with super-magnet). Granite 5 is the

second least weathered rock, but has the second highest Fe3+

/Fetot

ratio. Granite 5 has the

highest light rare earth element concentration of all the samples and therefore should have

significant amounts of monazite and/or allanite.

Granite 6

Grain size: 5-6 mm


Accessory minerals of significant concentration: Mn-almandine (alm59sp39gro2), monazite,

xenotime and amphibole (not confirmed with SEM-EDS analyses).

There is one distinct crack orientation with a diffuse crack orientation perpendicular to it. The

longest, widest and most connected cracks are situated in quartz and to a lesser extent in

plagioclase. This is a weak rock with a lot of wide open cracks. Biotites in Granite 6 are very

large (0.5mm) compared to the other rocks and almost all have large rusty haloes in hand

sample (Fig. 16.) Microcline is perthitic and some plagioclase is saussurised. Opaques are

small (0.02 mm) and very few compared to the other samples. 69% of the opaques are

inclusions in biotite, 19% are located far from biotites and 12% are situated adjacent to

biotites. Non-opaque inclusions in biotite are abundant and include monazite, ilmenite,

xenotime, apatite and zircon.

18

Fig. 16. Picture of granite 6.

Summary of the petrography

The texture and mineralogy differ considerably between the samples. All samples have lower

magnetite and biotite content than the Bohus granite. Most samples have few but large

biotites. Grain boundaries are more chaotic than in the Bohus granite. Heavy iron leakage is

almost exclusively found near biotite (Fig. 17.) and is most abundant in quartz cracks. No

sulphides were found in any of the samples. Samples with higher abundance of rare earth

element phosphates have many inclusions of such minerals in biotite compared to the Bohus

granite, which have no inclusions of rare earth element phosphates. Common inclusions in the

biotites of the Bohus granite are oxides and apatite. The normative quartz content in the

Bohus granite is higher than in any of the samples, but the quartz grains or aggregates are

usually larger than in the studied rocks. They also have a much higher crack density than the

quartz grains in the Bohus granite.

Fig. 17. Discolouration caused by iron leaking biotite. Left picture in plane polarized light and right picture in

cross polarized light. Pictures from Granite 1.

19

Discussion

A-granites have a low large ion lithophile elements/high field strength elements ratio and high

rare earth element content. None of these criteria are met. M-granites (mantle source granites)

have a low SiO2 content which seldom passes 70 wt.% and the Ca and Sr. All samples have

an A/CNK index below 1.1, less than 1% normative corundum and more than 3.2 wt.% Na2O

which indicate that they are I-type (igneous/infracrustal) granites (Chappell and White 2001).

The Bohus granite has characteristics of both I- and S-type (sedimentary/supracrustal) granite

(Eliasson 1987), but is generally considered to be an S-type granite. Magnetite is the

dominant oxide in I-granites, but the Bohus granite is rich in magnetite. In other words; these

rules are not “carved in stone”. The more fractionated the I- and S-granites become the more

their characteristics converge, which makes it more difficult to distinguish the two types.

Minerals characteristic of S-granites such as muscovite and garnet are found in some of the

samples. This is not a problem because I-granites become Al-saturated (peralumnious) with

fractionation. Garnet is formed at the expense of other minerals such as biotite because they

can accommodate more Mg, Fe and Mn and Al, without incorporating K, which has a

relatively low content in peralumnious rocks compared to Al. The compositions of major

elements change very little with fractionation because granites are close to the minimum melt

(Tuttle and Bowen 1958). The composition of trace elements varies considerably more with

fractionation. P2O5 increases with fractionation in S-type granites and decreases in I-types.

The highest P2O5 values are found in the least fractionated samples of this study, which is

another indication supporting the hypothesis that they are I-granites. The negative anomalies

of Ba, Nb, Sr, P, Eu and Ti and the positive anomaly for Pb in most samples (Fig. 7.) suggest

considerable fractional crystallization of common minerals such as plagioclase, K-feldspar

,biotite, ilmenite, titanite and apatite (Wu et al. 2003a). The similairities seen in granite

discrimination diagrams could mean that all granites are easily weathered for the same reason,

but the chemistry is probably an indirect cause. It is possible that the similar chemistry of

these rocks can be linked to structural features, responsible for their low weathering

resistance, but the relation is beyond the scope of this work. The classification does not

explain the low weathering resistance, but if more “bad granites” are classified one can find

out if certain granite types are weaker than others or not. Granite 1 and Granite 4 have very

similar whole-rock composition; both have hornblende and garnet, with almost identical

composition. The fact that their visual appearance is similar as well makes it very possible

that they both originate from Leizhou. The low rare earth element concentrations could be

caused by fractionation of monazite (Maruéjol et al. 1989). Heavy rare earth elements are

more mobile during weathering which means that the spider profiles of weathered rocks

should tilt steeper to the right if compared to unweathered samples (Yingjun and Congqiang

1999), but because there are no before and after weathering samples in this study it is not

possible to confirm.

The discolouration is clearly caused by iron leakage and to less extent by manganese in

at least one of the samples, from biotite. No clear examples of leaking opaques where found.

The relation between biotites and opaques are clearly different between the samples and is

probably not significant. The amount of non-opaque inclusions is very variable as well. The

Mn-rich mineralisations found in the cracks of Granite 3 are hard to explain because no Mn-

leaking mineral is found, but is probably due to biotites which are about 60% Mn-richer than

in the other samples. The Mn-ilmenite is not a probable source because ilmenite (FeTiO3) is a

very weathering resistant mineral (Easterbrook 1993). Weathering of garnets is poorly

studied, but based on ionic potentials spessartine should be least resistant to weathering

followed by almandine and pyrope. In general garnet is more resistant to weathering than

olivine but less than staurolite (Velbel 1999). Garnets are probably not an important source of

20

iron or manganese leakage in the initial stages of weathering, because no visible leakage was

found. Several samples have very disordered grain boundaries which could have been caused

by hydrothermal action, but it is hard to see how this affects the weathering resistance. The

rare earth element phosphates situated as inclusions in biotite could possible deform the

biotite when they weather, but no clear examples of this were found.

In many ways these rocks have chemical and mineralogical characteristics that may seem

good even compared to the Bohus granite such as low Fe and Mn content, less biotite and

magnetite and no iron sulphides. The biotites are very Fe- rich in all samples except Granite 3.

However Granite 3 is discolored anyway. The composition and weathering of biotite is

complex. The Fe3+

/Fetot

ratio in biotites were not determined in this study and could be

significant. It is also unclear how the manganese content affects the weathering resistance of

biotite. The weathering products and structural features of biotite were not studied as well.

The behavior of biotite during weathering in natural stones need further study. The nature of

defects, cracks and other textural features are much more likely to explain the fast weathering

rates of these rocks. The large cracks in quartz seen in many of the samples probably allow

water to penetrate the rock and oxidize the octahedral ferrous iron in biotite. The high amount

of cracks and defects increases the weathering surface substantially. The yellow granites

which are weathered before use are probably extracted from too shallow depths. In Swedish

quarries it is common practice to remove the first two meters of slightly weathered rock. In

e.g. some Spanish quarries the weathering zone is several tens of meters deep and

consequently removed before extraction of natural stone (pers. com. Angel Lopez 2009). In

southeastern China where the climate is almost tropical the depth to fresh rock should be a lot

deeper. It is possible that not enough weathered rock was taken away before some of these

rocks were extracted. It is possible that the mineralised cracks in the yellow granites might

give porosity values that do not reflect the condition of the fresh rock. The same is true for

crack analysis when counting open cracks. Maybe the amount of open cracks is low enough

but old healed cracks are often reopened. Even though the cracks become closed when

mineralised by weathering products they are often still a weakness. The physical strength of

iron hydroxides is probably lower than that of feldspars and quartz. The significance of cracks

and their connectivity, crystal defects and grain boundaries need to be studied further as they

probably are the major cause of the fast weathering rates of these rocks. The chemical,

mineralogical and visual similarities of Granite 1 and Granite 4 could be due to common

origin. There are no clear connection between grain size and grade of weathering.

Conclusions

All samples are classified as fractionated I-granites. The high fractionation explains the low

content of Ti, Mn, Ca, Mg and several trace elements, but how the rock quality is affected is

more obscure. It is possible that the shared fractionated I-type signature could explain the low

weathering resistance of these rocks, but the reasons are unclear in that case. It could explain

the high abundance of quartz and the quartz grains connectivity throughout the rock. All

samples are free of sulphides and have low concentrations of oxides. The chemical,

mineralogical and visual similarities of Granite 1 and Granite 4 could be explained by a

common origin. The discolouration is definitely caused by iron leakage due to weathering of

biotites. The biotites are iron rich for all samples except Granite 3 and they are all manganese

rich. The whole-rock content of Mn is low for all samples, but surprisingly concentrated in

minerals such as Mn-ilmenite, Mn-almandine and biotite. Minor Mn-leakage in Granite 3 is

most likely caused by the higher Mn content in biotite. It seems unlikely that direct chemical

reasons could explaining the fast weathering of these rocks, given the overall chemical

21

similarities in major elements compared to the Bohus granite. Three of the rocks are

discoloured due to weathering even before they are used and weathering is an exponential

process. The fast weathering of these rocks is most likely caused by their high density of

cracks, especially in quartz, and other defects. The cracks are often very long, wide and well

connected. It should be noted however that because only one fresh rock was studied except

the Bohus granite it is hard to determine the density of defects prior to the weathering, and it

would therefore be statistically wrong to exclude chemical reasons. Further chemical study is

needed on a large number of natural stones of both low and high quality in order to isolate

certain chemical and structural characteristics that signify granites with a discolouration

potential. It is important to have both fresh and weathered samples of the same rock in order

to determine the course of weathering. Especially the importance of chemical composition

and structural features of biotite, micro features and connectivity of cracks need further

studying. Information about the quarries is needed to in order to compare several rocks from

the same place. Circulation of water in paleocracks can cause slight weathering deep down

that is invisible to the naked eye, which change the quality of the rock substantially. The

quality can vary a lot over short distances.

Acknowledgements

This project was supported by the Cement and Concrete Research Institute and the University

of Gothenburg. The help from my supervisors David Cornell, Magnus Döse and Björn

Schouenborg are highly appreciated. Stellan Ahlin, Lennart Björklund, Malin Brandt, Mattias

Ek, Thomas Eliasson, Ali Firoozan, Fredrik Henriksson and Johan Hogmalm are thanked for

their support.

References

Bisdom, E. B. A., Stoops, G., Delvigne, J., Curmi, P., and Altemuller, H. J. (1982).

Micromorphology of weathering biotite and its secondary products. Pédologie 32:

225-252.

Boynton, W. V. (1984). Cosmochemistry of the rare earth elements: meteorite studies. In:

Hendersson, P. (ed.) Rare Earth Element Geochemistry. Amsterdam: Elsevier, pp. 63-114.

Chappell, B. W. (1999). Aluminium saturation in I- and S-type granites and the

characterization of fractionated haplogranites. Lithos 46: 535-551.

Chappell, B. W. and White, A.J.R. (2001). Two contrasting granite types: 25 years later. Aust. J. Earth Sci.

48: 489-499.

Dale, T. N. (1907). The granites of Maine: U. S. Geological Survey. Bulletin 313: 202 p.

De la Roche, H., Leterrier, J., Grandclaude, P. and Marchal, M. (1980). A classification of volcanic and

plutonic rocks using R1-R2-diagrams and major element analysis-its relationships with current

nomenclature. Chemical Geology 29: 183-210.

Deer, W. A., Howie, R.A. and Zussman, J. (1992). An Introduction to the Rock-forming Minerals. Harlow,

Pearson Education Limited.

Easterbrook, D. J. (1993). Surface processes and landforms. New York, Macmillan.

Eliasson, T. (1987). Trace element fractionation in the late Proterozoic Bohus granite, SW Sweden. Int.

Symp. on Granites and Associated Mineralizations, Extended Abstracts, Salvador, Bahia, pp, 211-214.

22

Eliasson, T., Ahlin, S. and Petterson, J. (1987). Emplacement mechanism and thermobarometry of the

Sveconorwegian Bohus granite, SW Sweden. GFF 125: 113-130.

Leake, B. E., Woolley, A. R., Arps, C. E. S., Birch, W. D., Gilbert, M. C., Grice, J. D., Hawthorne, F. C.,

Kato, A., Kisch, H. J., Krivovichev, V. G., Linthout, K., Laird, J., Mandarino, J. A., Maresch, W. V.,

Nickel, E. H., Rock, N. M. S., Schumacher, J. C., Smith, D. C., Stephenson, N. C. N., Ungaretti, L.,

Whittaker, E. J. W., and Youzhi, G. (1997). Nomenclature of amphiboles: Report of the Subcommittee

on Amphiboles of the International Mineralogical Association, Commission on New Minerals and

Mineral Names. The Canadian Mineralogist 35: 219-246.

Maniar, P. D. and Piccoli, P. M. (1989). Tectonic discrimination of granitoids. Geological Society of

American Bulletin 101: 635-643.

Maruejol, P., Cuney, M., and Turpin, L. (1989). Magmatic and hydrothermal R.E.E. fractionation in the

Xihuashan granites (SE China). Contributions to Mineralogy and Petrology 104: 668-680.

Pearce, J. A. (1983). Role or the sub-continental lithosphere in magma genesis at active continental

margins. In Continental Basalts and Mantle Xenoliths, ed. C.J, Hawkesworth and Norry, M.J., pp. 230-

249. Nantwich, UK: Shiva.

Pearce, J. A., Harris, N. B. W., and Tindle, A. G. (1984). Trace element discrimination diagrams for the

tectonic interpretation of granitic rocks. Journal of Petrology 25: 956-983.

Petersson, J. and Eliasson, T. (1997). Mineral evolution and element mobility during episyenitization

(dequartzification) and albitization in the postkinematic Bohus granite, southwest Sweden. Lithos 42:

123-146.

Pivko, D. (2004). World's quarries of commercial granites - localization and geology. Proceedings of the

International Conference in Dimension Stone, Prague, 1: 147-152.

Schouenborg, B., Grelk, B., Döse, M., Lindqvist, J-E. and Åkesson, U. (2010). Granites with discolouration

– How to avoid them. Globla Stone Congress.

Sun, S. -S., and McDonough, W.F. (1989). Chemical and isotopic systematics of ocean basalts:

Implications for mantle composition and processes. In Saunders, A.D., and Norry, M.J., Magmatism in

the Ocean Basins. Geological Society of London Special Publications., 42, 313-345.

Tuttle, O. F., and Bowen, N. L. (1958). Origin of granite in the light of experimental studies in the system

NaAlSi3O8-KAlSi3O8-SiO2-H2O. Geological Society of America Memoir 74.

Velbel, M. A. (1999). Bond strength and relative weathering rates of simple orthosilicates. American

Journal of Science 299: 679-696.

Whalen, J. B., Currie, K. L. and Chappell, B. W. (1987). A-type granites: Geochemical characteristics,

descrimination and petrogenesis. Contributionts to Mineralogy and Petrology 95: 407-419.

Wilson, M. J. (2004). Weathering of the primary rock-forming minerals: processes, products and rates.

Clay Minerals 39: 233-266.

Wilson, M. R. (1980). Granite types in Sweden. Geologiska Föreningens i Stockholm Förhandlingar 102:

167-176.

Winter, J. D. (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall, Upper Saddle

River.

Wu, F. Y., Jahn, B. M., Wilde, S. A., Lo, C. H., Yui, T. F., Lin, Q., Ge, W. C. and Sun, D. Y. (2003a).

Highly fractionated I-type granites in NE China (I): geochronology and petrogenesis. Lithos 66: 241-

273.

Yingjun, M. A. and Congqiang, L.I.U. (1999). Trace element geochemistry during chemical weathering.

Chinese Science Bulletin 44: 2260-2263.

23

Appendix

Table 1

Representative SEM-EDS analyses of microcline

Sample no. Granite 1 Granite 2 Granite 3 Granite 4 Granite 6

SiO2 (wt.%) 63.36 64.34 63.88 63.72 64.03

Al2O3 18.66 18.56 18.55 18.72 18.66

BaO 0.61 – – – –

Na2O 0.80 0.69 1.21 0.98 0.76

K2O 15.26 15.86 14.95 15.09 15.74

Total 98.69 99.45 98.59 98.51 99.19

Structural formula based on 8 O

Si 2.97 2.99 2.98 2.98 2.98

Al 1.03 1.02 1.02 1.03 1.02

Ba 0.01 – – – –

Na 0.07 0.06 0.11 0.09 0.07

K 0.91 0.94 0.89 0.90 0.94

∑ cations 4.99 5.01 5.00 5.00 5.01

Ab 7.14 6.00 11.00 9.09 6.92

Or 92.86 94.00 89.00 90.91 93.08

Table 2

Representative SEM-EDS analyses of plagioclase


SiO2 (wt.%) 63.94 61.61 63.74 64.58 65.07

Al2O3 22.11 24.00 21.75 21.83 21.07

CaO 2.92 5.11 2.73 2.70 1.96

Na2O 10.06 8.88 9.96 10.04 10.72

K2O 0.33 0.22 0.38 0.32 0.23

Total 99.36 99.82 98.56 99.47 99.05


Si 2.84 2.74 2.85 2.86 2.89

Al 1.16 1.26 1.15 1.14 1.10

Ca 0.14 0.24 0.13 0.13 0.09

Na 0.87 0.77 0.86 0.86 0.92

K 0.02 0.01 0.02 0.02 0.01

∑ cations 5.03 5.02 5.01 5.01 5.01

An 13.59 23.53 12.87 12.87 8.82

Ab 84.47 75.49 85.15 85.15 90.20

Or 1.94 0.98 1.98 1.98 0.98

24

Table 3

Representative SEM-EDS analyses of biotite


SiO2 (wt.%) 34.13 35.42 37.14 33.51 33.20

TiO2 4.45 2.79 2.62 2.65 1.39

Al2O3 15.00 15.49 15.98 14.74 17.88

ZnO 0.34 – – – –

FeO 26.84 26.18 16.94 30.19 29.95

MnO 1.08 0.84 1.67 1.07 1.11

MgO 3.73 6.63 10.59 3.21 2.03

Na2O – – – 0.35 –

K2O 9.19 9.52 9.62 8.83 9.04

Total 94.76 96.67 94.56 94.55 94.60


Si 5.50 5.54 5.67 5.50 5.42

Ti 0.54 0.33 0.30 0.33 0.17

Al 2.85 2.82 2.89 2.86 3.44

Zn 0.04 – – – –

Fe 3.62 3.43 2.17 4.15 4.10

Mn 0.15 0.11 0.22 0.14 0.17

Mg 0.90 1.55 2.42 0.80 0.50

Na – – – 0.11 –

K 1.89 1.90 1.87 1.84 1.87

Table 4

Representative SEM-EDS analyses of altered biotite and chlorite (biotite pseudomorphs)

Sample no. Granite 2 Granite 3 Granite 4 Granite 6

Mineral Chlorite Altered biotite Altered biotite Chlorite

SiO2 (wt.%) 24.27 33.34 34.75 24.49

TiO2 – – 1.94 –

Al2O3 18.95 18.51 15.30 19.68

ZnO – – 0,48 –

CoO 0.34 – – –

FeO 34.38 17.32 29.28 38.25

MnO 1.24 0.84 0.67 1.49

MgO 9.08 12.77 3.70 2.62

CaO – 0.16 – 0.23

Na2O – – – –

K2O – 3.63 3.46 0.25

Total 88.26 86.57 91.43 87.01

25

Table 5

Representative SEM-EDS analyses of typical Mn- and Fe-bearing accessory minerals


Mineral Mn-Almandine Magnetite Mn-Ilmenite Hastingsite Mn-Almandine

SiO2 (wt.%) 35.94 – – 37.63 34.98

TiO2 0.26 – 50.45 0.80 –

Al2O3 18.67 – – 11.38 20.13

V2O3 – 0.26 – – –

ZnO – – – – –

CoO – 0.54 – – –

FeO 20.55 92.48 32.52 29.73 25.99

MnO 14.41 – 15.44 1.59 16.96

MgO 0.48 – – 2.14 –

CaO 8.96 – – 10.30 0.65

Na2O – – – 1.84 –

K2O – – – 1.92 –

Total 99.27 93.28 98.41 97.33 98.71

Table 6

Representative SEM-EDS analyses of mineralised weathering products in cracks (yellowbrown mush)

Sample no. Granite 3 Granite 3 Granite 4

SiO2 (wt.%) 56.44 31.92 36.88

TiO2 – – 0.19

Al2O3 7.92 0.84 6.00

PbO – 1.28 –

ZnO – – –

FeO 20.52 – 39.24

MnO – 39.96 –

MgO – 0.61 0.25

CaO 1.59 0.49 –

BaO – 1.17 –

Na2O 4.39 0.82 –

K2O 0.37 2.12 0.38

Total 92.59 79.21 82.94

chemical characteristics of granites with a · in the natural stone industry the term granite has a...

Documents