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UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Chemical characteristics
of granites with a
discolouration potential
Joachim Andersson
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B631
Bachelor of Science thesis
Göteborg 2011
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
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
1
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
2
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
3
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+, Fe2+, Si4+, Ti4+, Fe3+ and Al3+. 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 Kfeldspar 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.
4
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.
5
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.
6
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
7
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).
8
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).
9
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).
10
Table 1 Chemical compositions
Sample no.
Rock condition
Granite 1
Weathered
Granite 2
Unweathered
Granite 3
Weathered
Granite 4
Weathered
Granite 5
Weathered
Granite 6
Weathered
SiO2 (wt.%)
TiO2
Al2O3
Fe2O3a
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
74.93
0.06
13.91
0.63
0.19
0.04
0.05
1.12
4.10
4.26
< L.D.
0.35
99.66
72.36
0.20
13.84
0.58
1.54
0.06
0.44
1.61
3.19
4.84
0.06
0.50
99.39
76.84
0.11
12.27
0.44
0.25
0.04
0.09
0.54
3.73
4.32
< L.D.
0.27
98.91
74.75
0.06
14.35
0.55
0.27
0.03
0.06
1.14
4.36
4.43
< L.D.
0.21
100.22
72.51
0.19
13.92
1.10
0.36
0.04
0.25
0.95
3.97
4.97
0.05
0.48
98.82
75.80
0.04
12.71
0.39
0.57
0.05
0.06
0.65
3.38
4.81
< L.D.
0.40
98.92
Cs (ppm)
Rb
Sr
Ba
Be
Nb
Ta
Th
U
Pb
Zr
Hf
Y
V
Co
Cr
Mo
F
Zn
Ga
Ge
Sn
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Fe3+/∑Fe
A/CNKb
Crnc
Qzd
Abd
Ord
0.287
98.68
329.1
900.6
1.115
3.563
0.162
2.766
0.820
33.33
77.11
2.496
8.315
1.160
< L.D.
4.075
< L.D.
90
34.46
17.08
1.022
0.737
6.178
12.32
1.509
5.817
1.363
0.378
1.130
0.171
1.128
0.271
0.938
0.168
1.235
0.210
0.75
1.04
0.52
35.05
37.64
27.31
2.041
132.5
97.75
563.7
1.228
8.686
0.777
14.02
4.120
26.41
129.2
3.906
17.06
18.86
2.156
4.385
< L.D.
350
48.18
16.46
1.500
1.728
32.64
69.45
7.973
28.64
5.165
0.616
3.637
0.505
2.769
0.519
1.508
0.232
1.609
0.243
0.25
1.03
0.57
36.05
31.05
32.90
2.003
121.8
79.08
545.6
2.926
10.44
1.117
9.838
2.278
26.22
98.45
3.654
20.86
3.211
< L.D.
7.005
0.443
200
< L.D.
14.65
1.578
1.350
23.61
50.48
5.660
19.38
3.683
0.500
2.969
0.498
3.027
0.605
1.777
0.295
2.045
0.319
0.61
1.04
0.48
39.53
33.43
27.04
0.333
129.3
264.9
773.1
1.326
3.209
0.158
3.687
0.876
42.78
81.69
2.654
8.030
0.968
< L.D.
5.945
0.33
130
47.15
19.00
1.186
0.697
6.623
11.53
1.585
5.903
1.415
0.324
1.217
0.176
1.049
0.215
0.634
0.102
0.724
0.115
0.65
1.02
0.31
32.16
39.68
28.16
1.987
190.9
218.2
657.7
4.499
23.26
2.398
25.64
4.167
39.58
208.6
5.976
14.87
9.858
1.295
5.210
0.428
550
39.72
20.40
1.436
2.288
56.60
97.08
9.164
27.55
3.902
0.596
2.556
0.378
2.120
0.402
1.215
0.199
1.483
0.248
0.73
1.02
0.40
30.88
36.88
32.24
4.197
204.0
26.29
176.1
1.557
10.54
1.068
19.86
3.898
35.18
70.54
2.881
56.16
1.579
< L.D.
5.384
< L.D.
120
24.67
15.98
1.718
3.351
14.85
32.51
4.117
16.19
4.827
0.239
6.038
1.146
7.970
1.852
5.658
0.891
5.975
0.935
0.38
1.06
0.76
38.86
30.66
30.48
a
Fe2O3=((Fe2O3tot×0.8998)-FeO)×1.1113
b
Al2O3/(CaO+Na2O+K2O) molar proportion.
c
Normative corundum (wt.%)
d
Normative quartz, albite or orthoclase (Qz+Ab+Or=100)
11
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).
12
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 Mnbearing 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
13
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 Feleaking 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.
14
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
Crack width: up to 0.04 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 Mnrich 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.
15
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.
16
Granite 4
Grain size: 1-3 mm
Crack width: up to 0.05 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
Crack width: up to 0.12 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.
17
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.
18
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 Mnleaking 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
19
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
20
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.
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22
Appendix
Table 1
Representative SEM-EDS analyses of microcline
Sample no.
Granite 1
Granite 2
Granite 3
Granite 4
Granite 6
SiO2 (wt.%)
Al2O3
BaO
Na2O
K2O
Total
63.36
18.66
0.61
0.80
15.26
98.69
64.34
18.56
–
0.69
15.86
99.45
63.88
18.55
–
1.21
14.95
98.59
63.72
18.72
–
0.98
15.09
98.51
64.03
18.66
–
0.76
15.74
99.19
2.99
1.02
–
0.06
0.94
5.01
6.00
94.00
2.98
1.02
–
0.11
0.89
5.00
11.00
89.00
2.98
1.03
–
0.09
0.90
5.00
9.09
90.91
2.98
1.02
–
0.07
0.94
5.01
6.92
93.08
Structural formula based on 8 O
Si
2.97
Al
1.03
Ba
0.01
Na
0.07
K
0.91
∑ cations
4.99
Ab
7.14
Or
92.86
Table 2
Representative SEM-EDS analyses of plagioclase
Sample no.
Granite 1
Granite 2
Granite 3
Granite 4
Granite 6
SiO2 (wt.%)
Al2O3
CaO
Na2O
K2O
Total
63.94
22.11
2.92
10.06
0.33
99.36
61.61
24.00
5.11
8.88
0.22
99.82
63.74
21.75
2.73
9.96
0.38
98.56
64.58
21.83
2.70
10.04
0.32
99.47
65.07
21.07
1.96
10.72
0.23
99.05
2.74
1.26
0.24
0.77
0.01
5.02
23.53
75.49
0.98
2.85
1.15
0.13
0.86
0.02
5.01
12.87
85.15
1.98
2.86
1.14
0.13
0.86
0.02
5.01
12.87
85.15
1.98
2.89
1.10
0.09
0.92
0.01
5.01
8.82
90.20
0.98
Structural formula based on 8 O
Si
2.84
Al
1.16
Ca
0.14
Na
0.87
K
0.02
∑ cations
5.03
An
13.59
Ab
84.47
Or
1.94
23
Table 3
Representative SEM-EDS analyses of biotite
Sample no.
Granite 1
Granite 2
Granite 3
Granite 4
Granite 6
SiO2 (wt.%)
TiO2
Al2O3
ZnO
FeO
MnO
MgO
Na2O
K2O
Total
34.13
4.45
15.00
0.34
26.84
1.08
3.73
–
9.19
94.76
35.42
2.79
15.49
–
26.18
0.84
6.63
–
9.52
96.67
37.14
2.62
15.98
–
16.94
1.67
10.59
–
9.62
94.56
33.51
2.65
14.74
–
30.19
1.07
3.21
0.35
8.83
94.55
33.20
1.39
17.88
–
29.95
1.11
2.03
–
9.04
94.60
5.54
0.33
2.82
–
3.43
0.11
1.55
–
1.90
5.67
0.30
2.89
–
2.17
0.22
2.42
–
1.87
5.50
0.33
2.86
–
4.15
0.14
0.80
0.11
1.84
5.42
0.17
3.44
–
4.10
0.17
0.50
–
1.87
Structural formula based on 22 O
Si
5.50
Ti
0.54
Al
2.85
Zn
0.04
Fe
3.62
Mn
0.15
Mg
0.90
Na
–
K
1.89
Table 4
Representative SEM-EDS analyses of altered biotite and chlorite (biotite pseudomorphs)
Sample no.
Mineral
SiO2 (wt.%)
TiO2
Al2O3
ZnO
CoO
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Granite 2
Chlorite
24.27
–
18.95
–
0.34
34.38
1.24
9.08
–
–
–
88.26
Granite 3
Altered biotite
33.34
–
18.51
–
–
17.32
0.84
12.77
0.16
–
3.63
86.57
24
Granite 4
Altered biotite
34.75
1.94
15.30
0,48
–
29.28
0.67
3.70
–
–
3.46
91.43
Granite 6
Chlorite
24.49
–
19.68
–
–
38.25
1.49
2.62
0.23
–
0.25
87.01
Table 5
Representative SEM-EDS analyses of typical Mn- and Fe-bearing accessory minerals
Sample no.
Mineral
Granite 1
Mn-Almandine
Granite 2
Magnetite
Granite 3
Mn-Ilmenite
Granite 4
Hastingsite
Granite 6
Mn-Almandine
SiO2 (wt.%)
TiO2
Al2O3
V2O3
ZnO
CoO
FeO
MnO
MgO
CaO
Na2O
K2O
Total
35.94
0.26
18.67
–
–
–
20.55
14.41
0.48
8.96
–
–
99.27
–
–
–
0.26
–
0.54
92.48
–
–
–
–
–
93.28
–
50.45
–
–
–
–
32.52
15.44
–
–
–
–
98.41
37.63
0.80
11.38
–
–
–
29.73
1.59
2.14
10.30
1.84
1.92
97.33
34.98
–
20.13
–
–
–
25.99
16.96
–
0.65
–
–
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.%)
TiO2
Al2O3
PbO
ZnO
FeO
MnO
MgO
CaO
BaO
Na2O
K2O
Total
56.44
–
7.92
–
–
20.52
–
–
1.59
–
4.39
0.37
92.59
31.92
–
0.84
1.28
–
–
39.96
0.61
0.49
1.17
0.82
2.12
79.21
36.88
0.19
6.00
–
–
39.24
–
0.25
–
–
–
0.38
82.94
25