identifying minerals in sedimentary rocks

8/8/2019 Identifying Minerals in Sedimentary Rocks

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Identifying minerals in thinsection

Common sedimentary rockminerals


2/10

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.


3/10

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.


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.


4/10

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)


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


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.


8/10

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)


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


10/10

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.

identifying minerals in sedimentary rocks

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