atlas of common rock-forming minerals

8/4/2019 Atlas of Common Rock-Forming Minerals

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Atlas of Common

Rock-Forming Mineralsin Thin Section

To be used in conjunction with

GEOL 303A and 303B laboratories

Brandon Browne

Department of Geological SciencesCalifornia State University, Fullerton


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Preface

This atlas has been prepared for undergraduate geology majors

who require a text for practical classes involving microscopeidentification of rock-forming minerals in igneous and

metamorphic rocks. In an effort to keep this book both simple and

widely useful, I limited the number of minerals described to 15. I

tried to present the text and pictures in a way so that students with

access to a petrographic microscope and a collection of thin

sections will find it useful. I have deliberately limited the scope of

the text to petrographic and textural characteristics because I

assume that the students using this book will already have some

background in the study of optical mineralogy- this especially goes

for you students who took 303A from me!! All pictures were taken

under relatively low magnification so as to illustrate general

mineral characteristics and inter-relationships with surrounding

minerals. I hope you find it useful. I will be making improvements

every year, so please send any suggestions to me

([email protected]).

Best Regards,

BLB

Fullerton, CA

2007

2
mailto:[email protected]:[email protected]


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ContentsThe 15 common rock forming minerals of this atlas:1. Olivine p. 4-5

2. Orthopyroxenes p. 6-73. Clinopyroxenes p. 8-11

Extinction angles of Pyroxenes p. 12

Two-pyroxene Intergrowth p. 13

4. Hornblende p. 14-17

5. Biotite p. 18-21

6. Muscovite p. 22-23

7. Chlorite p. 24-26

8. Quartz p. 27-30

9. Plagioclase Feldspar p. 31-34

10. Sanidine p. 35

11. Microcline p. 36-38

12. Orthoclase p. 39-40

13. Nepheline p. 41

14. Calcite p. 42

15. Garnet p. 43-44

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Plane Light, FOV = 1mm

Olivine

(Mg,Fe)2 SiO4

Olivine is thegeneral name given

to the solid solution

series betweenforsterite

(Mg2SiO4) and

Fayalite (Fe2SiO4).

Diagnostic Optical

Characteristics:

High relief3rd order

birefringence

No cleavage,instead

commonly

traversed byrandomly

oriented cracks

Occurrence:

Mafic andultramafic

igneous rocks

Commonly withCPX, OPX,

Plagioclase, Fe-

Ti oxides,Serpentine(alteration)

Rarely with Qtzor Kspar

Cross Polarized Light, FOV = 1mm

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Plane Light, FOV = 1 cm

Olivine

(Mg,Fe)2 SiO4

Olivine in Dunite/Peridotite rock

sample from

StrawberryOphiolite, central

Oregon.

Crossed Polarized Light, FOV = 1 cm

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Orthopyroxene

(Mg,Fe) SiO3

Orthopyroxene(OPX) is the general

name given to the

solid solution seriesbetween enstatite

(MgSiO3) and

Ferrosilite (FeSiO3).

Diagnostic Optical

Characteristics:

Mod. relief1st order grey,

yellow, and tanbirefringence

90 cleavageangles

Occurrence:

Mafic,ultramafic

igneous andmeta. (granulite

facies) rocks

Commonly withCPX, Olivine,

Plagioclase, Fe-

Ti oxides,Chlorite(alteration)


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Orthopyroxene

(Mg,Fe) SiO3

Orthopyroxene(OPX) with calcite

and feldspars.

Notice lowbirefringence and

90 cleavage.


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(Diopside in marble) Plane Light, FOV = 1mm

Clinopyroxene

Ca(Mg,Fe)

Si2O6

Clinopyroxene

(CPX) is the general

name given to theincomplete solution

series between

diopside(CaMgSi2O6),

Hedenburgite

(CaFeSi2O6) and

Augite

(Ca,Mg,Fe)2Si2O6.

Diagnostic OpticalCharacteristics:

Mod. relief2nd order red,

blue, blue-green,

and goldbirefringence

90 cleavageangles

Occurrence:

Mafic/ultramaficigneous rocks,

may also be in

diorites andgranodiorites

Commonly withOPX, Olivine,

Plagioclase, andFe-Ti oxides

(Diopside in marble) Cross Polarized Light, FOV = 1mm

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Clinopyroxene

Ca(Mg,Fe)

Si2O6

Diopside

surrounded by

hornblende andchlorite in gabbro

sample.


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Clinopyroxene

Ca(Mg,Fe)

Si2O6

Diopside (reds,

pinks, and blues in

XP) withplagioclase feldspar

(1st order grays) and

olivine (3rd orderbirefringence,

yellowish in PL) in

diabase sample

collected from Mt.

Baldy bowl, CA.


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Plane Light, FOV = 1 cm

Clinopyroxene

Ca(Mg,Fe)

Si2O6

Diopsite (reds,

pinks, and blues in

XP) with OPX (1st

order grays and

yellows) in

pyroxenite samplecollected from

Stillwater Intrusion,

MT.

Cross Polarized Light, FOV = 1 cm

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OPX has parallel extinction


Using

extinction

angles as a

means ofdistinguishing

pyroxenes:

CPX has inclined extinction


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Two-Pyroxene

Intergrowths

CPX sub-solus

exsolution

OPX sub-solvus

intergrowth


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Plane Light, FOV = 1 mm

Hornblende

NaCa2(MgFe)4

Al3Si6O22

(OH, F)2Hornblende is a member

of the amphibole group,

which contains a large

number of different

solid solutions but all

have a similar crystal

structure despite the

large amount of

compositional

variability from one

amphibole to another.

Diagnostic Optical

Characteristics:

Mod. relief2nd order red,

blue, blue-green,

and goldbirefringence

60,120 cleavageangles

Green, brown,tan, reddish,forms ofpleochroism

Commondisequilibria

(e.g., zoning orrxn rims) and

alteration

textures (e.g., tochlorite)

Occurrence:

Widely occursin virtually alltypes of igneous

rocks. Common

in metamorphicrocks too.

Cross Polarized Light, FOV = 1 mm

Hornblende in dacite porphyry, Icehouse Canyon, San Bernardino

Mtns

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Hornblende

NaCa2(MgFe)4

Al3Si6O22

(OH, F)2

Hornblende in

dacite porphyryaltering to chlorite

at rims (note changein birefringence at

rim compared to

core)


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Plane Light, FOV = 0.5 mm

Hornblende

NaCa2(MgFe)4

Al3Si6O22

(OH, F)2

Hornblende (green

mineral altering tochlorite) and titanite

(aka sphene) ingranodiorite sample,

Sierra Nevada

Cross Polarized Light, FOV = 0.5 mm

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Hornblende

NaCa2(MgFe)4

Al3Si6O22

(OH, F)2

Example of

hornblende (green,tan, and greenish

blue) pleochroismfrom Sierra Nevada

granodiorite sample.

Note 60 and 120

cleavage angles in

hornblende crystals.

Cross Polarized Light, stage rotated 60 degrees, FOV = 1 mm

Hbl

Hbl

Hbl

Hbl

Hbl

Hbl

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Biotite

K(MgFe)3

AlSi3O10

(OH, F)2

Diagnostic Optical

Characteristics:

Mod. relief2nd to 3rd order

birefringence,

but often

masked bybirds-eye

extinction

patternUniform

cleavage planes

Brown, tan,yellow, reddish

pleochroism

Often altered tochlorite

Occurrence:

Widely commonin nearly alligneous and

metamorphic

rocks, althoughrare in basalts or

andesites

Commonlyoccurs inclusters of Hbl

and Titanite

with chlorite

alteration


Biotite

Biotite

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Biotite

K(MgFe)3

AlSi3O10

(OH, F)2

Biotite in granite,

Sierra NevadaBatholith


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Biotite

K(MgFe)3

AlSi3O10

(OH, F)2

Biotite (tan and

brown) withmuscovite (colorless

in PL, 3rd

to 4th

order birefringence

in XP) surrounded

by Quartz andorthoclase in

granite, Sierra

Nevada Batholith


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Biotite

K(MgFe)3

AlSi3O10

(OH, F)2

Biotite with zircon

inclusions, denotedby the dark

pleochroic haloscaused by

radioactive decay of

U in zircons.

High magnification

of zircon inclusion

and pleochroic haloin biotite.


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Muscovite

KAl2

AlSi3O10

(OH, F)2

Diagnostic Optical

Characteristics:

Mod. relief3rd to 4th order

birefringence

Uniformcleavage planes

Colorless in PLBirds-eye

extinction in XP

Occurrence:

Widely commonin granitic andpegmatite rocks

and meta-pelitic

rocks (e.g.,schists, gneiss,

hornfels,

quartzite)

Rare in maficand ultramafic

rocks

Commonlyoccurs with Qtz,

Alkai feldspar

and biotite


Muscovite

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Muscovite

KAl2

AlSi3O10

(OH, F)2

Muscovite ingarnet-bearing

schist, San GabrielMtns


Garnet

Muscovite

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Chlorite

(Mg,Fe,Al)3

(Si,Al)4O10

(OH)2+(Mg,Fe,Al)3(OH)6

Diagnostic Optical

Characteristics:

Mod. relief1st order

birefringence,

may have

anomalous blueor green

birefringence

No cleavage-look for

rounded/shapeless crystal

faces

Green in PLOccurrence:

Widely commonin granitic andmeta-mafic

rocks (e.g.,greenschists,

meta basalts)

Rare inultramafic rocks

Commonlyalters epidote,biotite, hbl,

producing

titanite as abyproduct


Chlorite altering biotite and hornblende in granodiorite

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Chlorite

(Mg,Fe,Al)3

(Si,Al)4O10


Chlorite altering

hornblende andbiotite (producing

titanite) in

granodiorite from

San Bernardino

Mtns.

Note howhormblende, which

is already green in

PL, can bedistinguished from

Chlorite in XP due

to Chlorites lower

birefringence oranomalous

interference colorsand anhedral

morphology.


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Chlorite

(Mg,Fe,Al)3

(Si,Al)4O10


Chlorite altering

epidote ingranodiorite from

Mt. Baldy, San

Bernardino Mtns.

Note how epidote,which is yellow in

PL, can bedistinguished from

Chlorite in XP due

to Chlorites lowerbirefringence or

anomalous

interference colors

(epidote has 3rd

order birefringence)

and anhedralmorphology.


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Quartz

SiO2


Low to Mod.relief

1st orderbirefringence(gray and white)

No cleavage-look for

rounded faces involcanic rocks

Clear in PL

Uniaxial +Occurrence:

Widely commonin virtuallyevery rock type

Will not occurin mafic orultramafic rocks

Unstable inpresence of

olivine


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Quartz

SiO2Quartz (circled) in

granodiorite fromSan Bernardino

Mtns, with

hornblende

(altering to chloriteat rim), biotite, andfeldspar

Note low

birefringence,

amorphousmorphology, and

lack of cleavage


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Quartz


granite from SierraNevada Batholith

with hornblende

(altering to chlorite

at rim), biotite, Fe-Ti oxides (opaque)and feldspars.

Note low

birefringence,amorphous

morphology, and

lack of cleavage


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Quartz


dacite from Unzenvolcano, Japan is

embayed and

surrounded by

reaction rim ofCPX, plagioclase,and Fe-Ti oxides.

Note lowbirefringence,

amorphous

morphology, andlack of cleavage


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Plagioclase

Feldspar

NaAlSi3O8 -

CaAl2Si2O8

Diagnostic Optical

Characteristics:

Low to Mod.relief

1st orderbirefringence

(gray and white)

Albite and/orCarlsbad

twinningOscillatory

zoning if in

volcanic rocks

Clear in PLOccurrence:

Widely commonin virtually

every rock type

Not ultramaficrocks


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Plagioclase

Feldspar

NaAlSi3O8 -

CaAl2Si2O8

Plagioclase feldspar

displaying albitetwinning, appearing

as linear striationsthrough the crystal


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Plagioclase

Feldspar

NaAlSi3O8 -

CaAl2Si2O8

Plagioclase in

andesite lavaerupted from Mt.

Lassen, CA displaysoscillatory zoning


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Plagioclase

Feldspar

NaAlSi3O8 -

CaAl2Si2O8

Grain mount of

plagioclase chips.


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Sanidine in rhyolite porphyry (sill)


Sanidine

(K,Na)AlSi3O8


Low to Mod.relief

1st orderbirefringence(gray and white)

Tabular crystals Carlsbad

twinning

Clear in PL

Biaxial

Occurrence:

Only found insilicic or alkali-rich volcanic

rocks such as

rhyolite,

phonolite, ortrachyte.

May also occurin high Tmetamorphic

rocks of siliciccomposition

Not inultramafic or

mafic rocks

Sanidine in rhyolite lava


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Microcline

KAlSi3O8


Low to Mod.relief


(gray and white)

Tartan twinningClear in PLBiaxial

Occurrence:Occurs almost

exclusively ingranitic plutonic

such as granite,

monzonite,pegmatitic, and

syenite

Commonlyfound insedimentary

rocksNot in

ultramafic or

mafic rocks


36
http://www.science.smith.edu/departments/Geology/Petrology/Petrography/Sanidine/roll1.jpg


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Microcline

KAlSi3O8

Microcline showingtartan twinning,

which appears as a

cross-hatchedpattern in cross-

polarized light.


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Microcline

KAlSi3O8

Microcline showingtartan twinning,

which appears as a

cross-hatchedpattern in cross-

polarized light.


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Orthoclase

KAlSi3O8


Low to Mod.relief


(gray and white)

PerthiteExsolution

Texture

Clear in PL

Biaxial

Occurrence:

Occurs widelyin granitic

plutonic such asgranite,

granodiorite,

monzonite,

pegmatitic, andsyenite

Commonlyfound insedimentary

rocks

Not inultramafic or

mafic rocks


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Orthoclase

KAlSi3O8

Orthoclase ingranite from the

Sierra Nevada

Batholith.

Perthite (sub-

solvus)exsolution

texture as

observed in

cross-polarizedlight is a

diagnosticidentificationtool for

orthoclase in thin

section.

Note how albitetwinning in

plagioclase and

tartan twinningin microcline

compare to

perthite texture.


la ioclase

Orthoclase

microcline

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Nepheline

NaAlSiO4


Low to Mod.relief


(gray and white)

Tabular andprismatic habit

Often with notwinning

Clear in PL

Uniaxial Occurrence:

Occurs widelyin syenites and

related alkali-rich igneous

rocks

(feldspathoid-rich rocks like

phonolites)Commonly

found with

alkali feldspar

and plagioclase,biotite, leucite,

and Na anf K

ricj amphiboles

Not found withquartz


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Calcite

CaCO3


Low to Mod.relief

4th orderbirefringence

(appears gray-white except in

twinning planes)

RhombohedralTwinning

Colorless to tanPL

Uniaxial Occurrence:

Occurs widelyin sedimentary

and meta-

carbonate rocks

(e.g., marble)

Found in marblewith garnet,wollastonite,diopside,

epidote,

tremolite

Common as veinfilling mineral


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Garnet

(Mg,Fe)3

Al2Si3O12

Note- the chemical

composition of

garnet given above sa simplification.


High reliefIsotropic (zero

birefringence)

Poor cleavageOften occurs as

porphyroblastic

Occurrence:

Widely presentin moderate to

high grademetamorphic

rocks,

particularly

those with Al-rich protoliths

(with biotite,feldspar,

muscovite, and

quartz)

Also present insome mantle

xenoliths (with

olivine+OPX)Cross Polarized Light, FOV = 1mm

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Garnet

(Mg,Fe)3

Al2Si3O12

Garnet with biotite

(note zircon halos),

muscovite, andquartz in schist from

Catalina Island.


atlas of common rock-forming minerals

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