minerals under microscope
TRANSCRIPT
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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
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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
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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.
http://www.geolab.unc.edu
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.
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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
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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.
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
Oli i
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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.
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
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
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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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Andalusite (And) Al2SiO5
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)
Andalusite (And) Al2SiO5
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
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
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Kyanite (kyanite-garnet- mica schist)
Note the prismatic habit of kyanite
http://www.geolab.unc.edu
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.
http://www.geolab.unc.edu
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
http://www.geolab.unc.edu
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Calcite Note the rhombohedral cleavage and very
high order interference colors.
http://www.geolab.unc.edu
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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
http://www.geolab.unc.edu
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
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
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
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
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
PerthiteExsolution
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ppl
xpl
Streaky,orientedalbite
inclusions inmicrocline
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
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
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
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
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
Sphene (Titanite) CaTiSiO5
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xpl
http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/ig_minerals.htm
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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http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm
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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http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm
Cordierite xpl
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http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm
Cordierite xpl
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http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm
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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