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Identifying minerals in thinsection
Common sedimentary rockminerals
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Quartz SiO 2
not visible in thin sectionTwinning
noneAlteration Productsanhedral granular crystals whichare highly irregular
CrystalForm
none2Vnone; displays conchoidalfracture
Cleavage
positiveOptic Signlow positiveRelief
first-order white/gray (yellow possible especiallyif the thin section is too thick)
Interference Colours(birefringence)
colourless/noneColour/Pleochroism
possibly undulatoryExtinctionanisotropic-uniaxialLight Behavior
Properties
Quartz is a tectosilicate that displays low relief, low birefringence and lackscleavage and twinning.
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lamellar-parallel or oblique to long diagonal or parallel tothe edges of the cleavage rhombs
Twinning
dolomite ?Alteration Productscrystals are rhombohedral, although they are usuallyanhedral
Crystal Form
152Vthree rhombohedralCleavage
negativeOptic Signmoderate to high negative-changes with rotationRelief
creamy high-order colours.
Interference Colours(birefringence)
white or colourless/noneColour/Pleochroism
inclined or symmetrical
Extinctionanisotropic-uniaxialLight Behavior
Properties
Calcite: CaCO 3
Calcite is a carbonate which displays high birefringence, a change in relief with
rotation, polysynthetic twin lamellae (common), and rhombohedral cleavage.
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Dolomite CaMg(CO 3)2
lamellar-parallel to long or short
diagonalTwinning
may be pseudomorphically replaced byother carbonates
Alteration Productscrystals are rhombohedral, and theyare usually euhedral
Crystal Form
none2V perfect-rhombohedralCleavage
negativeOptic Signlow negative to high positive-changes with rotation
Relief
creamy high-order white or grayInterference Colours(birefringence)
colourless, may have brownishcolour/none
Colour/Pleochroism
inclined or symmetricalExtinctionanisotropic-uniaxialLight Behavior
Properties
Dolomite is a carbonate that displays high birefringence, a change in relief withrotation, twin lamellae, rhombohedral cleavage, and is usually euhedral. It is
common in sedimentary rocks.
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Orthoclase KAlSi3O
8
Orthoclase is a k feldspar that has low relief, inclined extinction ,low birefringence, and a larger 2V angle than sanidine. It is common in
intrusive igneous rocks; it also occurs in some metamorphic rocks.
carlsbadTwinning
sericite or clay (kaolinite)Alteration Productscrystals are tabular parallel to (010), elongate parallel to c or a-axis
Crystal Form
40-702Vtwo intersect at 90 degreesCleavage
negativeOptic Signlow negativeRelief
first-order whiteInterference Colours(birefringence)
colourless, commonly clouded/noneColour/Pleochroism
inclined, parallel if cut perpendicular to (010)
Extinctionanisotropic-biaxialLight Behavior
Properties
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Plagioclase
CaAl 2Si 2O 8(albite) - NaAlSi 3O 8(anorthite)
Plagioclase is a tectosilicate that has low relief, inclined extinction,and displays polysynthetic twinning.
albite, carlsbad, or periclineTwinning
sericite, saussuriteAlteration Productscrystals are tabular parallel to {010}, elongate parallel tothe c or a-axis, x-sections are rectangular, zoning iscommon.
Crystal Form
45-1022Vtwo intersect at 90Cleavage
neg. or pos.Optic Signlow negative or positiveRelief
first-order white/gray(yellow when morecalcic)
Interference Colours(birefringence)
colourless, may be clouded or noneColour/Pleochroism
inclinedExtinctionanisotropic-biaxialLight Behavior
Properties
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Microcline KAlSi 3O 8
tartanTwinning
sericite or clay
Alteration Productscrystals are tabular parallel to {010}, elongate parallel toc or a-axis
Crystal Form
65-882Vtwo intersect at 90Cleavage
negativeOptic Signlow negativeRelief
first-order white
Interference Colours(birefringence)
colourless, may be clouded/noneColour/Pleochroism
inclinedExtinctionanisotropic-biaxialLight Behavior
Properties
Microcline is a K feldspar that displays tartan twinning, and hasinclined extinction, low relief, and low birefringence.
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Muscovite KAl2(AlSi
3O
10)(OH)
2
rarely visible in thin sectionTwinning
clay mineralsAlteration Productscrystals are micaceous flakes with an irregular outline, tabular parallel to (001), rarely euhedral with hexagonal outline
CrystalForm
30-472Vone perfect cleavageCleavage
negativeOptic Signmoderate positive-slight change with rotationRelief
second or third-order (vividcolours)
InterferenceColours(birefringence)
colourless/none; exhibits "bird's eye maple" extinctionColour/Pleochroism
parallelExtinctionanisotropic-biaxialLight
Behavior
Properties
Muscovite is a phyllosilicate that has high birefringence, lacks colour under ppl, and has one cleavage direction.
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BiotiteK(Mg,Fe) 3AlSi 3O 10 (OH,O,F) 2
rarely visible in thin sectionTwinning
chlorite, sometimes clayminerals, magnetite
Alteration Productseuhedral, tabular crystals parallel to {001}, with aroughly hexagonal x-section, sometimesmicaceous grains with irregular form.
Crystal Form
0-252Vone perfect cleavageCleavage
negativeOptic Signmoderately high positive, low if Mg-richRelief
third or fourth-order (may be masked byits colour)
Interference Colours(birefringence)
brown, brownish-green, reddish-brown/light tan or pink to shades of brown and green; exhibits
Colour/Pleochroism
parallelExtinctionanisotropic-biaxialLight Behavior
Properties
Biotite is a phyllosilicate that displays pleochroism, parallel extinction, and one perfect cleavage direction
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Chlorite(Mg,Al,Fe)
3(SiAl)
4O
10(OH)
2(Mg,Al,Fe)
3(OH)
6
composition planes on {001} are difficultto recognize
Twinning
fairly resistant, some clay-minerals, iron-oxides
Alteration Productscrystals are commonly platy (as inmicas), they are rarely tabular
parallel to (001)
Crystal Form
0-40/0-602Vone perfect on {001}Cleavage
negative or positiveOptic Signlow to moderateRelief
first-order white or yellow; anomalous purple or brown colours possible
InterferenceColours(birefringence)
light to medium green/colourless toshades of green
Colour/Pleochroism
parallel (maximum of 9)Extinctionanisotropic-biaxialLight Behavior
Properties
Chlorite is a phyllosilicate which is colourless to green, may display pleochroism,with a low birefringence.
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