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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
Best Regards,
BLB
Fullerton, CA
2007
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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
3
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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
4
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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
5
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Plane Light, FOV = 1mm
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)
Cross Polarized Light, FOV = 1mm
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Plane Light, FOV = 1mm
Orthopyroxene
(Mg,Fe) SiO3
Orthopyroxene(OPX) with calcite
and feldspars.
Notice lowbirefringence and
90 cleavage.
Cross Polarized Light, FOV = 1mm
7
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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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Plane Light, FOV = 1mm
Clinopyroxene
Ca(Mg,Fe)
Si2O6
Diopside
surrounded by
hornblende andchlorite in gabbro
sample.
Cross Polarized Light, FOV = 1mm
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Plane Light, FOV = 1mm
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.
Cross Polarized Light, FOV = 1mm
10
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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
11
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OPX has parallel extinction
Cross Polarized Light, FOV = 1mm
Using
extinction
angles as a
means ofdistinguishing
pyroxenes:
CPX has inclined extinction
Cross Polarized Light, FOV = 1mm
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Cross Polarized Light, FOV = 1mm
Two-Pyroxene
Intergrowths
CPX sub-solus
exsolution
OPX sub-solvus
intergrowth
Cross Polarized Light, FOV = 1mm
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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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Plane Light, FOV = 1 mm
Hornblende
NaCa2(MgFe)4
Al3Si6O22
(OH, F)2
Hornblende in
dacite porphyryaltering to chlorite
at rims (note changein birefringence at
rim compared to
core)
Cross Polarized Light, FOV = 1 mm
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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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Plane Light, FOV = 1 mm
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
17
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Plane Light, FOV = 1 mm
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
Cross Polarized Light, FOV = 1 mm
Biotite
Biotite
18
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Plane Light, FOV = 1 mm
Biotite
K(MgFe)3
AlSi3O10
(OH, F)2
Biotite in granite,
Sierra NevadaBatholith
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
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
Cross Polarized Light, FOV = 1 mm
20
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Plane Light, FOV = 1 mm
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.
Cross Polarized Light, FOV = 0.25 mm
21
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Plane Light, FOV = 1 mm
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
Cross Polarized Light, FOV = 1 mm
Muscovite
22
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Plane Light, FOV = 1 mm
Muscovite
KAl2
AlSi3O10
(OH, F)2
Muscovite ingarnet-bearing
schist, San GabrielMtns
Cross Polarized Light, FOV = 1 mm
Garnet
Muscovite
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Plane Light, FOV = 1 mm
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
Cross Polarized Light, FOV = 1 mm
Chlorite altering biotite and hornblende in granodiorite
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Plane Light, FOV = 0.5 mm
Chlorite
(Mg,Fe,Al)3
(Si,Al)4O10
(OH)2+(Mg,Fe,Al)3(OH)6
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.
Cross Polarized Light, FOV = 0.5 mm
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Plane Light, FOV = 1 mm
Chlorite
(Mg,Fe,Al)3
(Si,Al)4O10
(OH)2+(Mg,Fe,Al)3(OH)6
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.
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Quartz
SiO2
Diagnostic OpticalCharacteristics:
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
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Quartz
SiO2Quartz (circled) in
granodiorite fromSan Bernardino
Mtns, with
hornblende
(altering to chloriteat rim), biotite, andfeldspar
Note low
birefringence,
amorphousmorphology, and
lack of cleavage
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Quartz
SiO2Quartz (circled) in
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
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 0.25 mm
Quartz
SiO2Quartz (circled) in
dacite from Unzenvolcano, Japan is
embayed and
surrounded by
reaction rim ofCPX, plagioclase,and Fe-Ti oxides.
Note lowbirefringence,
amorphous
morphology, andlack of cleavage
Cross Polarized Light, FOV = 0.25 mm
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Plane Light, FOV = 1 mm
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
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Plagioclase
Feldspar
NaAlSi3O8 -
CaAl2Si2O8
Plagioclase feldspar
displaying albitetwinning, appearing
as linear striationsthrough the crystal
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 0.5 mm
Plagioclase
Feldspar
NaAlSi3O8 -
CaAl2Si2O8
Plagioclase in
andesite lavaerupted from Mt.
Lassen, CA displaysoscillatory zoning
Cross Polarized Light, FOV = 0.5 mm
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Plane Light, FOV = 1 mm
Plagioclase
Feldspar
NaAlSi3O8 -
CaAl2Si2O8
Grain mount of
plagioclase chips.
Cross Polarized Light, FOV = 1 mm
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Sanidine in rhyolite porphyry (sill)
Cross Polarized Light, FOV = 1 mm
Sanidine
(K,Na)AlSi3O8
Diagnostic OpticalCharacteristics:
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
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Microcline
KAlSi3O8
Diagnostic OpticalCharacteristics:
Low to Mod.relief
1st orderbirefringence
(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
Cross Polarized Light, FOV = 1 mm
36
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Plane Light, FOV = 1 mm
Microcline
KAlSi3O8
Microcline showingtartan twinning,
which appears as a
cross-hatchedpattern in cross-
polarized light.
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1 mm
Microcline
KAlSi3O8
Microcline showingtartan twinning,
which appears as a
cross-hatchedpattern in cross-
polarized light.
Cross Polarized Light, FOV = 1 mm
38
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Plane Light, FOV = 0.5 mm
Orthoclase
KAlSi3O8
Diagnostic OpticalCharacteristics:
Low to Mod.relief
1st orderbirefringence
(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
Cross Polarized Light, FOV = 0.5 mm
39
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Plane Light, FOV = 1 mm
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.
Cross Polarized Light, FOV = 1 mm
la ioclase
Orthoclase
microcline
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Cross Polarized Light, FOV = 1 mm
Nepheline
NaAlSiO4
Diagnostic OpticalCharacteristics:
Low to Mod.relief
1st orderbirefringence
(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
Cross Polarized Light, FOV = 1 mm
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Cross Polarized Light, FOV = 0.5 mm
Calcite
CaCO3
Diagnostic OpticalCharacteristics:
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
Cross Polarized Light, FOV = 1 mm
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Plane Light, FOV = 1mm
Garnet
(Mg,Fe)3
Al2Si3O12
Note- the chemical
composition of
garnet given above sa simplification.
Diagnostic OpticalCharacteristics:
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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Plane Light, FOV = 1mm
Garnet
(Mg,Fe)3
Al2Si3O12
Garnet with biotite
(note zircon halos),
muscovite, andquartz in schist from
Catalina Island.
Cross Polarized Light, FOV = 1mm