Minerals Under Microscope

8/3/2019 Minerals Under Microscope

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Mineralsunder the microscope

compiled by

Dr.Pete Kinny &Dr.R.Nagarajan


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The MINERALS you need to know

quartz microcline

plagioclase

muscovite

biotite

amphibole

pyroxene

olivine garnet

calcite


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Plagioclase (Pl) NaAlSi3O8 to CaAl2Si2O8

Colourless, low relief, equant xenoblastic or

granoblastic grains, but may be tabular and wellformed if relict igneous minerals. Lowbirefringence (grey interference colours),characteristic multiple twinning, but may havesimple twins (particularly if albite). Commonlyalters to fine grained white mica (sericite).

Common in low- to high-grade metapelites andmetabasites at low to intermediate pressure(generally more calcic with increasingtemperature). Not stable in high pressure

blueschist or eclogite facies rocks.


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Quartzgrains in sandstone

XPLPPL

quartz

quartz

quartz

quartz

http://www.geolab.unc.edu


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Quartz

This slide shows quartz in a range of crystalorientations, all having low-first order interferencecolors.



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Sub-grains in quartz The crystal structure of this quartz grain has

been deformed (probably by low-grade

metamorphism) to produce sub-grains.



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Quartz after coesite qtz-ky-gt-musc schist The region of coarser-grained quartz in the upper center portion of this

photomicrograph was probably originally occopied by coesite, the high-pressure polymorph of quartz. Metamorphic rocks from the Dora MairaMassif show other evidence of being exhumed from EXTREMELY deeplevels in thickened crust.



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Coesite qtz-ky-gt-musc schist Coesite (center of inclusion) and recrystallized quartz (borders of inclusion) form a

tiny inclusion in nearly pure endmember pyrope garnet from the famous Dora Mairamassif of Italy. The presence of coesite (a high-pressure polymorph of quartz)

indicates that this rock saw extremly high pressures during metamorphism (probablymore than 28 kbar).



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Minerals in Thin Section

(Rock)


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Quartz


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Quartz (Qtz) SiO2

Colourless, low relief, equant xenoblasticor granoblastic grains, low

birefringence (grey interference colours),low relief, "clean" appearance,

uniaxial positive, may show unduloseextinction.

Common in all lithologies except ultramaficrocks.


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Plagioclasewith repeated twinning

XPLPPL

plagioclase

plagioclase



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Plagioclase This slide showcases one of plagioclase's very

common features: its polysynthetic twinning. Contrastthis with twinning in microcline (K-feldspar).

http://www.geolab.unc.ed
http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.html


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Plagioclase This plagioclase is a xenocryst in a vitrophric

volcanic rock. Note the compositional zoning

and the fractured portion of the crystal.



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Plagioclase


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Plagioclase


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Plagioclase


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But, note that some minerals are better magicians than others(i.e., some grains stay dark and thus cant be reorienting light)

olivine

plag

PPL XPL


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Piemontite Piemontite has beautiful, high-order interference colors.



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Piemontite Piemontite has beautiful rose to yellow pleochroism.



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Piemontite


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Piemontite

K feldspar (Kfs) KAlSi3O8 to NaAlSi3O8


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K-feldspar (Kfs) KAlSi3O8 to NaAlSi3O8

Colourless, equant xenoblastic or granoblastic

grains, but may be tabular and well-formed ifrelict igneous minerals. Low birefringence (greyinterference colours), sometimes with "tartan"

twinning (microcline) or stringy intergrowths(perthite), difficult to tell from plagioclase if bothuntwinned, may alter to fine-grained white mica(sericite).

Common in metapelites at high grade, andpsammites and metagranites at all grades. Rare

in metabasites, absent in ultramafic rocks.


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Microclinegrain in centre showing its distinctive cross-

hatched twinning

XPLPPL

microcline microcline



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Microcline (K-feldspar) Cross-hatched (or "tartan") twinning in microcline. Contrast this

with polysynthetic twinning in plagioclase feldspar.http://www.geolab.unc.edu
http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/plagtwins.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/plagtwins.X.html


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Microcline


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Perthite


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Muscovite (Ms) KAl3Si3O10(OH)2

Colourless platy crystals, one good basal

cleavage, high birefringence

(brightly coloured yellow, pink, blue or greeninterference colours), speckled extinction.Common in low to medium-grade

metapelite, meta-granite, and quartzite.


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Muscoviteclear under PPL but with perfect cleavage and

bright interference colours

XPLPPL

muscovite

muscovite

PPL XPL



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Muscovite

This grain is shown at maximum birefringnence. Take a look at this grain at extinction to see muscovite's

wonderful "bird's eye" mottling.

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/muscovite.EXT.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/muscovite.EXT.html


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Muscovite Bird's eye" mottling in muscovite grain at extinction.



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Muscovite


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Muscovite


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Sericite (a fine-grained variety of muscovite)

The feldspars in this alaskite from the Boulder Batholith have

been largely replaced by fine-grained muscovite (sericite). Inthis rock, sericite is a product of hydrothermal alteration.


Bi tit (Bt) KM 3Si3AlO10(OH)2 t


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Biotite (Bt) KMg3Si3AlO10(OH)2 toKFe3Si3AlO10(OH)2

Brown or reddish brown to straw yellowpleochroic crystals (darkest colour when length

of lath is parallel to the polarising filter), withplaty to tabular shape and one good basalcleavage. High birefringence (green or pinkinterference colours) with "speckled" extinction.

Common in all rock types, except at highpressure.


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Biotitepleochroic with one perfect cleavage; strong

natural colour masks polarisation coloursmaking look greenish under XPL

XPL

PPL : lower view rotated to show pleochroism

biotite

biotite

biotite

feldspar

feldspar


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Biotite (garnet-mica schist) Note the anomalous red

interference color.Note the red-brown pleochroism andthe characteristic black pleochroichaloes (caused by radiation damage).


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Biotite


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Biotite with Haloes


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Biotite with Haloes


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Biotite


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Color andpleochroism

Biotite


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Phlogopite


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Phlogopite


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Amphibolepleochroic with 2 cleavages at 60oor 120odistinguishing it from biotite or pyroxene

XPL

PPL : lower view showing pleochroism

amphibole

amphibole


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Pyroxenewell-formed crystal seen here with 2 cleavages at 90o:

brightly coloured under XPL

XPLPPL

pyroxene

pyroxeneenlarged


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Olivinelooks like a stained-glass window; bright interference

colours and thick black fractures

PPL XPL

olivineolivine

plagioclase plagioclase

Color and pleochroism


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Color is observed only in PPL Not an inherent property - changes with light type/intensity

Results from selective absorption of certainl

of light Pleochroism results when different l are absorbeddifferently by different crystallographic directions -rotate stage to observe

plag

hbl

plag

hbl

-Plagioclase is colorless-Hornblende is pleochroic in olive greens

Color and pleochroism

Slides from Jane Selverstone (University of

New Mexico) and John Winter (WhitmanCollege)


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Vesuvianite Vesuvianite displays the deepest indigo blue

anomalous interference colors you've ever seen!http://www.geolab.unc.edu


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Vesuvianite


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Tremolite Talc forms the fine-grained matrix between the prismatic crystals of tremolite

in this rock. Note the ~120 degree cleavages in some of the tremolitesections.



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Tremolite


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Tremolite


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Clinopyroxene (Cpx) CaMgSi2O6 to CaFeSi2O6

High-relief colourless to weakly pleochroic pale green elongate toequant grains (more intense colour if more Fe-rich). Two goodcleavages (at 90) in cross section. Moderate to high birefringence(grey, orange, blue, green interference colours) inclined extinction

(up to 45).

Mg-rich end-member diopside common in impure marbles andcalcsilicates, Fe-rich end-member hedenbergite common in

skarns and metaironstones, intermediate Fe-Mg salite commonin high-grade metabasites.


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Augite (Clinopyroxene)clinopyroxenitehttp://www.geolab.unc.edu

Note the pigeonite twin lamellae in this grain.Pigeonite is a Ca-poor clinopyroxene.

Besides clinopyroxene's 2nd order colors, anotherway to distinguish clinopyroxene from orthopyroxene

is by clinopyroxene's inclined extinction.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/cpx.EXT.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/cpx.EXT.html


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Augite (Clinopyroxene) Note the pigeonite twin lamellae in this grain.

Pigeonite is a Ca-poor clinopyroxene.

Augitewith


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Exsolution

gpigeoniteexsolution

lamellae

Pigeonite is aCa-poorclinopryoxene

Exsolution in pyroxene

Left: http://www.geosci.unc.edu/Petunia/IgMetAtlas/template.htmlRight: http://jaeger.earthsci.unimelb.edu.au/Images/Mineralogical/html/Im68.html


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Diopside


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Aegerine (Acmite)


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Aegerine (Acmite)


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Orthopyroxene (Opx) Mg2Si2O6 to

Fe2Si2O6

High-relief colourless to pale pink pale greenpleochroic equant grains. Two

good cleavages (at 90).

Low to moderate birefringence (grey, orange,

blue interference colours), straight extinction.

Intermediate FeMg hypersthene common inhigh-grade metabasites


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Hypersthene Orthopyroxenes are noted for having low, first-order

interference colors. Also note the cleavages that intersect atabout 90 degrees. Another identifying characteristic of

orthopyroxene is its parallel extinction.
http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/opx.EXT.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/opx.EXT.html


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Hypersthene Hypersthene can be identified from its characteristic rose-colored pleochroism

which is exhibited by the grain in the center. Watch what happens to the color of this grain upon rotation of the stage.

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/opx.UX.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/opx.UX.html


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Hypersthene After the stage is rotated about 45 degrees, the

grain takes on a slightly greenish hue.



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Hornblende (Hbl) Ca2Mg4Si7Al2O22(OH)2

to Ca2Fe4Si7Al2O22(OH)2

Clinoamphibole. Green to blue green or olive green or brownelongate prismatic crystals (more brown and less elongate with

increasing grade), with two good cleavages in cross section (at56).

Moderate to high birefringence (blue, green, orange

interference colours), inclined extinction (typically up to 25)and length slow.

Common in medium to high-grade metabasites.



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Hornblende Note the characteristic ~120 degree cleavage angles in some

sections and the brown to green. pleochroism.


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Hornblende (hornblende-cpx schist) Note the characteristic ~120 degree cleavage angles in some

sections and the brown to green. pleochroism.



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Cleavage

2 cleavagesintersectingat ~90pyroxene

601202 cleavagesintersectingat 60/120:amphibole

Slides from Jane Selverstone (Univ. of New Mexico) and John Winter (Whitman College)


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Extinction angle parallel extinction

All uniaxial minerals show parallel extinction Orthorhombic minerals show parallel extinction

(this is because xtl axes and indicatrix axes coincide)

PPL XN

orthopyroxene

Slides from Jane Selverstone (University of New Mexico) and John Winter (WhitmanCollege


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Extinction angle parallel extinction

All uniaxial minerals show parallel extinction Orthorhombic minerals show parallel extinction

(this is because xtl axes and indicatrix axes coincide)

PPL XN

orthopyroxene

Slides from Jane Selverstone (University of New Mexico) and John Winter (WhitmanCollege


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Hornblende


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Hornblende


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Hornblende

Glaucophane (Gln) Na2Mg3Al2Si8O22(OH)2


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Glaucophane (Gln) Na2Mg3Al2Si8O22(OH)2to Na2Fe3Al2Si8O22(OH)2

Clinoamphibole.Pale blue to lilac to pale yellow or colourless

elongate grains or needles, with two goodcleavages in cross section (at 56).Moderate birefringence (grey, orange, blueinterference colours), inclined extinction (up

to 15) and length slow.

Common in high pressure metabasites and

metasediments.


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Glaucophane(blueschist)

Note the anomalous blue-gray interference

colors in the glaucophane in this slide.



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Glaucophane

Note the purple to blue-graypleochroism in the glaucophane in

this slide.


G


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Glaucophane

Gl h


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Glaucophane


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Olivine Almost all of the grains in this rock are olivine. Note the high

order interference colors and the minor secondary calcite.


Oli i
http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/calcite.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/calcite.X.html


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Olivine

Oli i


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Olivine

L Oli i G bb


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Lunar Olivine Gabbro

L Oli i G bb


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Lunar Olivine Gabbro


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Nepheline (nepheline basalt)

Many of the phenocrysts in this basalt are nepheline.


Tourmaline (Tur)NaMg3Al6B3Si6(O OH)30(OH F)


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NaMg3Al6B3Si6(O,OH)30(OH,F)NaFe3Al6B3Si6(O,OH)30(OH,F)

Pleochroic deep blue green to olive or pink,elongate prisms, hexagonal or triangular incross-section. The strongest colour is when the

length of the grain is perpendicular to thepolarising filter (unusual). Moderate to highbirefringence (orange, pink, blue, green)

straight extinction. Fe end-member schorl occurs in all rock

types, particularly if metasomatic. Mg end-

member dravite less strongly coloured.


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Tourmaline This slide shows extinct trigonal cross-sections and

elongate sections displaying maximum birefringence.

The matrix is quartz.



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Tourmaline This slide shows zoned trigonal cross-sections and

elongate sections. Note green to brown pleochroism.Tourmaline may also display bluish pleochroism.


Sphene (Spn) (or Titanite)


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p ( p ) ( )CaTiSiO4(O,OH,F)

High-relief, small pale brown crystals, xenoblastic orwedge shaped. No cleavage. Very high birefringence(very pale interference colours and mineral is the

same colour is cross polarised light as in planepolarised light.

Often forms a rim around ilmenite or rutile.

Common in medium-grade metabasites, and marblesand calc-silicates. Rare in pelites.


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Titanite

Titanite typically forms wedge-shaped crystals like this one. Alsonotice the extremely high interference colors.



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Titanite Titanite typically forms wedge-shaped crystals like this

one. Also notice the extremely high relief.


Sphene (Titanite)


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Sphene (Titanite)

Sphene (Titanite)


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Sphene (Titanite)

Sphene (Titanite)


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Sphene (Titanite)

Sphene (Titanite)


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Sphene (Titanite)

Nepheline


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Nepheline

Nepheline


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Nepheline

Cordierite (Crd) Mg2Al4Si5O18 toFe2Al4Si5O18


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Colourless, low relief, equant xenoblastic or granoblastic grains,commonly with a dusty appearance. Low birefringence (grey to

very pale yellow interference colours).

May show sector or multiple twinning with some twins at 60, andhas yellow pleochroic halos around radioactive inclusions.

Often altered to pale yellow pinite at grain margins and along

fractures.

Common in medium-grade pelites at low pressure and high-grademetapelites at medium and low pressure.

Fe2Al4Si5O18

Cordierite


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Cordierite

Cordierite


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Cordierite

Epidote


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Epidote

Epidote


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Epidote

Epidote


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Epidote

Talc (Tlc) Mg3Si4O10(OH)2


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Talc (Tlc) Mg3Si4O10(OH)2

Similar to muscovite (colourless,platy, speckled interference) but

has higher birefringence (morewashed out interference colours).

Common in metamorphosedultramafic rocks.


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Talc Talc forms the fine-grained matrix between the prismatic crystals of

tremolite in this rock. Note the fine-grain size and the extremely high

interference colors of the talc.


Talc


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Talc

Talc


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Talc


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Stilpnomelane

Stilpnomelane looks alot like biotite (same habit and color) butlacks the "bird's eye" extinction that biotite displays. Actually,the stilpnomelane in this slide has a more acicular habit than

most biotite.

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/biotite.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/biotite.X.html


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Stilpnomelanehttp://www.geolab.unc.edu

Stilpnomelane


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St p o e a e

Stilpnomelane


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p a

Garnet (Grt) Pyralspite: Mg3Al2Si3O12 toFe3Al2Si3O12 to Mn3Al2Si3O12


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High relief equant, idioblastic to rounded xenoblasticcolourless to very pale pink porphyroblasts. Usuallyisotropic (zero birefringence).Pyralspite series (pyrope-almandine-spessartine)

common in pelites (almandine-rich) and metabasiceclogites (pyrope rich).Ugrandidte: Ca3Cr2Si3O12 to Ca3Al2Si3O12 toCa3Fe2Si3O12

Ugrandite series (uvarovite-grossular-andradite)common in impure marbles and skarn rocks. Somehydrated forms of grossular not isotropic(hydrogrossular).


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XPLPPL

Garnetrounded crystal,transparent under PPL but goes

extinct under XPL

garnetgarnet


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Garnet (garnet-mica schist)

Note the zonal distribution of quartz inclusions in this garnetporphyroblast.

Garnet


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Garnet

Garnet


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Garnet

Garnet


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Garnet

Garnet


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Garnet

Garnet


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Garnet


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Epidote (Ep) Ca2(Al,Fe3+)3Si3O12(OH)

High relief, yellow to yellow-green pleochroism, elongate toxenoblastic grains that generally lack a good cleavage. High

birefringence (bright pink, yellow, green, blue interference colours,commonly zoned from core to rim).

Common in medium-grade metabasites and calc-silicates.


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Epidote (Biotite granite gneiss)

Note the high-order interferencecolors of epidote. This slide isactually cut a little thin, anddoesn't show the third-ordercolors that epidote may

display in some sections.

Note the fairly high relief ofepidote, the highly fracturedmineral in this photomicrograph.

Andalusite (And) Al2SiO5


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Colourless (rarely very pale pink), high relief, elongateidioblastic prisms to rounded xenoblastic grains, often formsporphyroblasts and may be full of quartz inclusions. May showone or two (~90) cleavages if well formed.

Low birefringence (grey interference colours), straightextinction, length fast, and may have characteristic darkinclusions in a diagonal cross pattern (chiastolite). Commonlyaltered to fine-grained white mica (sericite).

Common in medium-grade pelites at low pressure.


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Andalusite This is an andalusite porphyroblast with poikiloblastic texture. Also note how

the foliation (oriented roughly N-S in this view) is wrapped around the leftand right corners of this grain, suggesting synkinematic growth of the

andalusite porphyroblast. http://www.geolab.unc.edu

Garnet (Nesse, Ch. 16, p. 310-314)
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/quartzmicaschist.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/quartzmicaschist.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.html


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MR-3: garnet in gneissicamphibolite (6.25 mm)

PPL

XN

PPL

XN

OM-6: garnet in sillimaniteschist (4 mm)


Colourless (rarely very pale pink), high relief, elongate idioblastic prisms to roundedxenoblastic grains often forms porphyroblasts and may be full of quartz inclusions May


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xenoblastic grains, often forms porphyroblasts and may be full of quartz inclusions. Mayshow one or two (~90) cleavages if well formed.Low birefringence (grey interference colours), straight extinction, length fast, and may

have characteristic dark inclusions in a diagonal cross pattern (chiastolite). Commonlyaltered to fine-grained white mica (sericite).Common in medium-grade pelites at low pressure.

Sillimanite (Sil) Al2SiO5

Colourless, high relief, fibrous (fibrolite) or needle like grains. may show one diagonalcleavage in square cross sections. Moderate birefringence (grey, orange, blueinterference colours), straight extinction and length slow.Common in high-grade pelites.

Kyanite (Ky) Al2SiO5

Colourless, high relief, bladed subidioblastic to idioblastic crystals, with very well-developed cleavages (at ~80) that typically appear very dark. Low to moderatebirefringence (higher than andalusite but lower than sillimanite, grey to orangeinterference colours), inclined extinction (up to 30) and length slow.

Common in medium- rade elites at medium to hi h ressure.

Al2SiO5(Nesse, Ch. 16, p. 314-319)

Al SiO i l i thi ti ERTH 2001 ll ti


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OM-13:andalusite(var. chiastolite) 2 mm

PPLPPLPPL

XN XNXN

OM-6:sillimanite4 mm

OM-4:kyanite4 mm

Al2SiO5 minerals in thin section, ERTH 2001 collection

Kyanite (Ky) Al2SiO5


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Colourless, high relief, bladed subidioblastic toidioblastic crystals, with very well-developedcleavages (at ~80) that typically appear verydark. Low to moderate birefringence (higher

than andalusite but lower than sillimanite, greyto orange interference colours), inclinedextinction (up to 30) and length slow.

Common in medium-grade pelites at medium tohigh pressure.

Kyanite


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y

Kyanite


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y

Kyanite


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y

Kyanite


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y

Sillimanite (Sil) Al2SiO5


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Colourless, high relief, fibrous (fibrolite)or needle like grains. may show one

diagonal cleavage in square crosssections. Moderate birefringence (grey,

orange, blue interference colours),

straight extinction and length slow.Common in high-grade pelites.

Sillimanite


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Sillimanite


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Sillimanite


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Sillimanite


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Sillimanite


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Sillimanite


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Staurolite (St) FeAl9Si4O22(OH) toMgAl9Si4O22(OH)


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MgAl9Si4O22(OH)

Pale yellow to yellow to colourlesspleochroic porphyroblasts, often full of

quartz inclusions and prismatic in shape.Poor cleavage and often formscruciform twins. Low to moderate

birefringence (grey to orangeinterference colours) and straight extinction.Common in medium grade pelites.

Staurolite


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Staurolite


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Staurolite


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Staurolite


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Staurolite The "swiss cheese" look (i.e., poikiloblastic texture) of

these staurolite porphyroblasts is typical for this mineral.

Also note the strong banana yellow pleochroism


http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.html


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Staurolite (staurolite-kyanite -garnet-mica schist)

The "swiss cheese" look (i.e., poikiloblastic texture) ofthese staurolite porphyroblasts is typical for thismineral. Also note the strong banana yellow

pleochroism
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/poikiloblastic.X.html


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Kyanite (kyanite-garnet-

mica schist)

Note the first-orderinterference colorsand prismatic habit ofkyanite.



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Kyanite (kyanite-garnet- mica schist)

Note the prismatic habit of kyanite


Chloritoid (Ctd) Fe2Al4Si2O10(OH)4 to

M 2Al4Si2O10(OH)4


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Mg2Al4Si2O10(OH)4

Pale yellow to blue-green pleochroic

elongate prismatic grains. Often

packed with inclusions and may show hour-glass structures and multipletwinning. Moderate birefringence (but

typically has anomalous blueinterference colours).Common in low to medium-grade pelites.


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Chloritoid These stubby crystals are chloritoid

porphyroblasts. You can just barely seethe anomalous green interference color atthe edge of some of the grains.

Despite the name, chloritoid really doesn't

look anything like chlorite.

This plane-light view highlights thesector zoning in these chloritoidporphyroblasts.


Chlorite (Chl) Mg4Al4Si2O20(OH)8 to

Fe4Al4Si2O20(OH)8
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/chlorite2.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/chlorite2.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/porphyroblastic.X.html


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Fe4Al4Si2O20(OH)8

Pale green to colourless or pale yellowpleochroism, may form platy

crystals, has anomalous inference colours (dark

brown if Mg-rich, purple or dark blue if Fe-rich, or grey), often forms as a

replacement of higher grade

minerals like biotite and garnet. Common in low-grade pelites, metabasites and

other rock types.


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Chlorite (chlorite schist)

Chlorite defines the foliation in thisrock, which also shows somecrenulation cleavage.

In this photo, a large "book" of red-brownbiotite is partially replaced by greenchlorite (in granite)

Chlorite (granite)http://www.geolab.unc.edu
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/quartzmicaschist.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/crenulation.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/biotite.UX.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/biotite.UX.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/crenulation.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/quartzmicaschist.X.html


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Chlorite replacing biotiteWhen replacing biotite (brown), chlorite (green) typically appears to "spread apart" thesheets of the biotite structure.

Chlorite


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Chlorite


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Chlorite


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Chlorite


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Chlorite


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Chlorite


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Sericite (a fine-grained variety of muscovite)Rock type Alaskite

The feldspars in this alaskite from the Boulder Batholith have beenlargely replaced by fine-grained muscovite (sericite). In this rock,sericite is a product of hydrothermal alteration.

http://www geolab unc edu


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XPLPPL

Calcitemay show both high and low relief, transparent under PPL with2 perfect cleavages at 120o. Under XPL it shows pearly shades

calcite calcite



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Calcite Note the rhombohedral cleavage and very

high order interference colors.



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Calcite vein This vein of calcite was formed as fluids entered an open-space

fracture during alteration of this rock. Notice how the orange and red

grains of kaersutite (amphibole) match up across the fracture


Petrographically simple(?)Bi tit


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XPL 20X PPL 20X

Halifax Pluton, Novia Scotia, 375 Ma, photos copyright K. Hollocher, Geol.Dept.

Union College N Y

Plagioclase

Plagioclase

Quartz

Quartz

K-spar

K-spar

Biotite

Biotite

Biotite


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Ellenbergeriteqtz-ky-gt-musc schist

The dark reddish-purple grain in the center of this photomicrographis ellenbergerite, an extremely rare, high-pressure Mg-Al-Ti-silicate,which here forms an inclusion in nearly pure endmember pyropegarnet from the famous Dora Maira massif of Italy.

Poikiloblastic texture - tourmaline-mica schist


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Poikiloblastic texture describes porphyroblasts which are riddled with finergrained inclusions of other minerals. Here, orange tourmaline and gray K-feldspar include numerous, fine- grained quartz and muscovite mica grains.

http://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/tourmaline.UX.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/quartz.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/muscovite.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/muscovite.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/quartz.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/microcline.X.htmlhttp://www.geolab.unc.edu/Petunia/IgMetAtlas/minerals/tourmaline.UX.html


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Minerals in thin section


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1. Ferromagnesian mineralsin igneous rocks

Mg Fe

Forsteritic (Mg) Olivine


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Colourless in ppl Anhedral shape

resorbed phenocrysts in basalts,

granular aggregates in peridotites,

spinifex texture in komatiites

High relief

Prominent fractures no cleavage

Common alteration to serpentine, etc 2nd order interference colours in xpl

pplOlivine


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xplOlivine


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Serpentine (antigorite) replacing olivine ppl


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Serpentine (antigorite) replacing olivine xpl

Fayalitic (Fe) Olivine


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Similar to forsteriticolivine but yellow -green in ppl

High relief (R.I. > Mg-

olivine) 3rd order interference

colours in xpl ( >Mg- olivine)

May co-exist withquartz

ppl

Clinopyroxene - Augite


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Prismatic shape, lacking elongation Pale pink, green, mauve or brown in ppl

Titanaugite darker colour, zoned, pleochroic

Moderately high relief (< olivine)

Pyroxene cleavage (2 @ 87 on basal section)

2nd order interference colours ( < olivine) in xpl

Inclined extinction

Simple twins common


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Augite ppl


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xplAugite

Clinopyroxene - Diopside


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CaMgSi2O6 Similar to augite, but

colourless in ppl

Rich green colour if

Cr-bearing

Occurs in someultramafic rocks

ppl

Ca-poor Clinopyroxene - Pigeonite


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Similar toorthopyroxene, buthas higherbirefringence and

inclined extinction Occurs in some

basalt lavas

ppl

xpl

Na-Clinopyroxene - Aegirine


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Strongly pleochroic yellow green in ppl Elongate shape (unusual for a pyroxene)

Moderately high relief

Pyroxene cleavage (2 @ 87 on basalsection)

3rd order xpl colours ( > augite)

Nearly straight extinction Simple twins common


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Aegirine ppl


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Aegirine xpl

Orthopyroxene - Enstatite

C l l l h i


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Colourless to pleochroic

pale green - pink in ppl Blocky prismatic shape Moderately high relief

(similar to cpx) Pyroxene cleavage (2 @

87 on basal section) Straight extinction Simple twins common Exsolution lamellae of

cpx common 1st order xpl colours ( feldspars)

Undulose extinction (when strained) Uniaxial, positive optic sign

Quartz


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xpl

ppl

Clean-looking Quartz among dirty-looking Alkali Feldspar


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ppl

http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm

Plagioclase Feldspar


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Transparent - colourless in ppl Ca-plagioclase typically lath-shaped

Low positive relief (Ca-plag. R.I. > quartz)

Alteration to sericite, etcdirty-looking

Cleavage seldom visible in thin section 1st order grey interference colours in xpl

Multiple polysynthetic twinning - Albite law

Variable extinction angle - visible growth zoning Biaxial optic sign & 2V vary with Ca/Na ratio

Andesine

Intermediate


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ppl

xpl

Intermediate

Na/CaPlagioclaseFeldspar

Bytownite

Ca-rich


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Ca rich

PlagioclaseFeldspar

ppl

xpl

Sericitealterationof


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ppl

xpl

ofPlagioclaseFeldspar


Alkali Feldspar

T t l l i l
http://www.union.edu/PUBLIC/GEODEPT/http://www.union.edu/PUBLIC/GEODEPT/


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Transparent - colourless in ppl

Blocky phenocrysts, elongate in trachytes

Low negative relief (R.I. < resin)

Alteration commondirty-looking

Cleavage not always visible in thin section

1st order grey interference colours in xpl

Simple twinning - Carlsbad law in sanidine & orthoclase

Cross-hatched multiple twinning in microcline

Perthite exsolution common (Albite lamellae)

Graphic intergrowths with quartz common

Biaxial optics, negative sign (2V orthoclase > sanidine)

Sanidine

KAlSi3O8


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KAlSi3O8

High-TMonoclinic

AlkaliFeldspar

Occurs in volcanic

rocks

ppl

xpl

Orthoclase

KAlSi3O8


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KAlSi3O8

Lower-TMonoclinic

Alkali Feldspar

Occurs in shallowplutonic /

hypabyssal rocks

ppl

xplhttp://www.union.edu/PUBLIC/GEODEPT

/COURSES/petrology/ig_minerals.htm

Microcline

KAlSi3O8


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xpl

3 8

TriclinicAlkali

Feldspar

Occurs inplutonic

igneous and inmetamorphicrocks

ppl

Microcline tartan twinning Albite + Pericline Laws


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xpl


PerthiteExsolution


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ppl

xpl

Streaky,orientedalbite

inclusions inmicrocline


Anorthoclase

(K Na)AlSi O


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(K,Na)AlSi3

O8

IntermediateK/Na

TriclinicAlkaliFeldspar

Occurs in volcanicrocks

ppl

xpl

Myrmekite - worm-like quartz inclusions in plagioclase


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xpl


Feldspathoids

Nepheline NaAlSiO4


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Nepheline NaAlSiO4

Transparent - colourless in ppl Low positive relief similar to quartz In granular aggregates, rarely as hexagonal crystals

Dusty appearance similar to feldspars Alters to cancrinite (a CO3-bearing feldspathoid

recognisable by its 2nd order interference colours) 1st order grey interference colours ( < quartz) Uniaxial negative Characteristic of silica-poor alkali-rich rocks

- Incompatible with quartz


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Feldspathoids

Leucite

ppl


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Leucite

KAlSi2O6

Trapezohedral crystals(pseudo-isometric) Low negative relief Multiple twinning

Low 1st order greyinterference colours

In lavas onlyxpl

Muscovite

Transparent

ppl


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Transparent

colourless in ppl

Platy habit

Moderate relief

One perfect cleavage 3rd order interference

colours in xpl

Birds-eye mapleextinction

xpl

Calciteppl


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CaCO3

xpl

Colourless in ppl

Relief varies with

rotation

Rhombohedral

cleavage

Multiple twinning Extreme birefringence

Uniaxial negative

Fluorite ppl


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CaF2

xpl

Isometric crystals

Colourless topurple in ppl

Moderate negative

relief

Octahedral cleavage

Isotropic


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3. Accessory minerals

Zircon ZrSiO4


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ppl

Zircon ZrSiO4


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xpl

Can restite be identified?

Zircon!!!!

Very resistant mineral which survives


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From Paterson et al. (1992), Trans. Royal. Soc.

Edinburgh. 83, 459-471. Also Geol. Soc. Amer.Spec. Paper, 272, 459-471.

erosion, sedimentation, high grademetamorphism and partial melting AND

can be dated in situ.

Zircon from Strontian Granite, Scotland,

showing a rounded, unzoned core (inherited

from source) and igneous overgrowth(s)crystallised from the pluton.

Zircon


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Zircon


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Tourmaline


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Tourmaline


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Tourmaline


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Allanite (REE-Epidote)


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ppl

Allanite (REE-Epidote)


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xpl

Apatite (Ca-phosphate)


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ppl

Apatite (Ca-phosphate)


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xpl

Sphene (Titanite) CaTiSiO5


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ppl


Sphene (Titanite) CaTiSiO5


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xpl


Monazite (REEphosphate)


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ppl

Monazite (REEphosphate)


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xpl


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Minerals in thin section


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4. Metamorphic minerals


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Photo:PeteKinny


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Andalusite sieve texture of poikiloblast

ppl


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Photo:Pete Kinny

Andalusite (Chiastolite) showing cleavage


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xpl

http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm

Sillimanite

Al2SiO5

ppl


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Colourless, transparent Acicular or fibrous habit Moderate relief (R.I. 1.66) 1 cleavage + basal parting Interference colours in xpl

up to 2nd order blue Straight extinction Length slow (optically +ve)

xplhttp://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm

Sillimanite

(Fibrolite) ppl


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Photo:Pete Kinny

Kyanite

Al2SiO5

ppl
http://www.union.edu/PUBLIC/GEODEPT/http://www.union.edu/PUBLIC/GEODEPT/http://www.union.edu/PUBLIC/GEODEPT/http://www.union.edu/PUBLIC/GEODEPT/http://www.union.edu/PUBLIC/GEODEPT/


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Colourless, transparent Bladed triclinic crystals High relief, similar to garnet 2 cleavages @ 80 Interference colours in xpl

up to 1st order red Inclined extinction Length fast

Biaxial, negative

Photos: Pete Kinny

Colourless in ppl, unless Fe-rich (Dichroite),which is strongly pleochroic, colourless to violet

Cordierite (Mg,Fe)2Al4Si5O18 . n(H2O)


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Low relief similar to quartz Usually anhedral, rarely as hexagonal crystals Dusty appearance similar to feldspars

Alters to pinite, a yellowish-coloured mixture ofchlorite, micas, talc, etc Yellow pleochroic halos around zircon inclusions Birefringence similar to quartz

Multiple polysynthetic and cyclic twinning Biaxial with high 2V, usually negative In low-P metamorphic rocks and some granites

Cordierite ppl


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Cordierite xpl


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Cordierite xpl


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Staurolite sieve texture of poikiloblast

ppl


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Photo:Pete Kinny

Pleochroic - shades of green Moderate positive relief

Chlorite ppl


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Platy (micaceous) habit One perfect cleavage Low birefringence

Anomalous interferencecolours in xpl- blue, purple, brown

In low grade & retrogressed

metamorphic rocks, oftenreplacing biotite

xplPhotos: Pete Kinny

Chlorite

l


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Anomalousinterferencecolours

Photos: Pete Kinny

xpl

Pleochroic shades ofgreen (lighter than

h bl d )

Actinolite ppl


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hornblende)Elongate prisms

Moderate positive relief

Amphibole cleavage2nd order interferencecolours in xpl

In low grade metamorphicrocks, and as uralite

alteration of igneouspyroxenes

www.ucl.ac.uk

Pleochroic - shades ofyellow to yellow-green

Hi h iti li f

Epidote ppl


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High positive relief Granular or elongate habit One cleavage, not always

visible in thin section

3rd order interferencecolours in xpl

In metamorphic and

Minerals Under Microscope

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