Metamorphic rocks in thin section

Birefringence is first order white to pale yellow, much like quartz and feldspars but with higher relief and very different habit even when not big, blocky crystals... Plane polarized li

METAMORPHIC ROCK IN THIN

SECTION

Contents Image scales (field width)

Al-silicates Other aluminous minerals Amphiboles

Pyroxenes Sheet silicates Epidotes Oxides

20x = 6 mm 40x = 3 mm 100x = 1 mm 200x = 0.5 mm 400x = 0.25 mm 1000x = 0.1 mm

Al-silicates

Andalusite in a muscovite-biotite schist Note the diamond-shaped inclusion cloud to

the bottom left, which is the center of the crystal, and the inclusion trails that radiate fromthe corners of the diamond This is known a "chiastolite cross" Note the small garnet just to the below the diamond Two {110} cleavages intersecting at approximately right angles are visible Andalusite has lower relief than the other Al-silicates

Plane polarized light, 20x

NEIGC84-C5-2

Andalusite in a muscovite-biotite schist Note the isotropic garnet, and the inclusion-rich

area at the top-right Birefringence is first order white to pale yellow, much like quartz and feldspars but with higher relief and very different habit (even when not big, blocky crystals)

Cross polarized light, 20x

NEIGC84-C5-2

Kyanite in a muscovite-biotite schist The four kyanite crystals are colorless, have high

relief, and two have a strong cleavage parallel to their length Relief is much higher than muscovite, which surrounds the kyanite

Plane polarized light, 40x

TMW96-C4b

Kyanite in a muscovite-biotite schist The kyanite crystals are have interference colors

up to upper 1st order, much lower than the surrounding muscovite Most sections yield slightly inclined extinction, as expected from its triclinic symmetry

Cross polarized light, 40x

TMW96-C4b

Sillimanite fibers (variety fibrolite) in a biotite-andalusite-muscovite schist Sillimanite is

colorless, has relief much higher than muscovite In medium-grade rocks sillimanite is typically in this fibrous variety Sillimanite fibers can be included in many minerals, and can survive retrograde metamorphism in garnet and quartz

Plane polarized light, 200x

JBT2-XA

Sillimanite fibers (variety fibrolite) in a biotite-andalusite-muscovite schist Sillimanite

has birefringence up to 2nd order blue, somewhat higher than kyanite and much higher than andalusite Extinction is parallel as required by its orthorhombic symmetry These fibers are so thin they scarcely have any birefringence

Cross polarized light, 200x

JBT2-XA

Sillimanite prisms in a biotite-garnet-cordierite schist This granulite facies schist has

very coarse sillimanite prisms which can be seen in long section and in their shaped cross sections

diamond-Plane polarized light, 40x

WE-1

Sillimanite prisms in a biotite-garnet-cordierite schist The lower second order blue

birefringence can be seen in the longitudinal sections

Cross polarized light, 40x

WE-1

Sillimanite and andalusite in a biotite-muscovite-andalusite schist The large high-relief

(grayish) patches are andalusite, which are surrounded by a coarse muscovite rim (lower right) The andalusite is partially pseudomorphed by sillimanite, which are high-relief, colorelss diamond shapes

Plane polarized light, 20x

JBT2-XA

Sillimanite and andalusite in a biotite-muscovite-andalusite schist In this view the

andalusite are the large gray areas (mostly), and the sillimanite pseudomorphing andalusite are the vertically elongated white diamonds scattered throughout the upper-left half of the image

Cross polarized light, 20x

JBT2-XA

Other aluminous minerals

Cordierite in a garnet-cordierite-biotite schist Cordierite is colorless, and relief is similar

to plagioclase feldspars and quartz, but it tends to be dustier than either of these other two minerals

Plane polarized light, 40x

WE-1

Cordierite in a garnet-cordierite-biotite schist Birefringence is up to 1st order white, like plagioclase and quartz, and can have polysynthetic twinning (center) that resembles plagioclase Note that there can be THREE directions of polysynthetic twinning,

contrasting with none in quartz and typically two in feldspars

Cross polarized light, 40x

WE-1

Cordierite in a garnet-cordierite-biotite schist Magnified view of yellow pleochroic halos

surrounding radioactive inclusions

Plane polarized light, 200x

WE-1

Cordierite in a garnet-cordierite-biotite schist The pleochroic halos around radioactive

inclusions are typically brownish or purplish in cross polarized light

Cross polarized light, 40x

WE-1

Staurolite in a muscovite-biotite schist Staurolite has parallel extinction and one

cleavage parallel to its length, and has a characteristic pale to darker golden yellow pleochroism This example has a twin, highlighted by different color and birefringence caused by its different orientation

Plane polarized light, 20x

Gassetts Schist

Staurolite in a muscovite-biotite schist The birefringence of staurolite is similar to that of

kyanite, upper 1st order

Cross polarized light, 20x

Gassetts Schist

Chloritoid in a muscovite-chlorite phyllite The pale blue to yellow pleochroism of the

chloritoid, and its high relief, contrasts sharply with the lower relief, pale yellow to green pleochroic chlorite This grain has prominent twinning, evident in parallel stripes having somewhat different pleochroic colors

Plane polarized light, 20x

IMA96-C1-1

Chloritoid in a muscovite-chlorite phyllite The low first-order birefringence is somewhat

higher than that of most chlorite, and much lower than muscovite The polysynthetic twinning and the somewhat anomalous first order interference colors of chloritoid are visible

Cross polarized light, 20x

IMA96-C1-1

Chloritoid in a muscovite-chlorite phyllite Closeup showing green chlorite between

bluer and much higher relief chloritoid

Cross polarized light, 100x

IMA96-C4B

Tourmaline in a muscovite-biotite schist Tourmaline occurs as elongate to stubby

prisms having hexagonal or triangular cross sections They are typically zoned in shades

of blue, green, or brown

Plane polarized light, 200x

East Clairindon, VT

Tourmaline in a muscovite-biotite schist Tourmaline has no cleavage (though

commonly cross fractures), and has negative elongation

Cross polarized light, 200x

East Clairindon, VT

Tourmaline in a muscovite-biotite schist Tourmaline is unusual among common

elongate minerals in having its strongest absorption when the plane of light polarization

is perpendicular to the crystal length This is the opposite of micas and most amphiboles.Plane polarized light, 200x

East Clairindon, VT

Tourmaline in a muscovite-biotite schist Tourmaline is pale-colored when the plane of

polarization is parallel to its length

Plane polarized light, 200x

East Clairindon, VT

Amphiboles

Actinolite in a greenstone These actinolite crystals are very pale green, and here occur

as stubby crystals These are probably pseudomorphs after augite phenocrysts in the basalt protolith

Plane polarized light, 40x

NNH-3

Actinolite in a greenstone Like most monoclinic amphiboles actinolite has birefringence

in the lower 2nd order Twinning, possibly relic from the original augite, can be seen.Cross polarized light, 40x

NNH-3

Cummingtonite in a hornblende - biotite - cummingtonite amphibolite These crystals

are almost colorless, but have abundant hornblende exsolution lamellae in them that are

on irrational planes approximately parallel to {100} and {001}

Plane polarized light, 100x

Q-603C

Cummingtonite in a hornblende - biotite - cummingtonite amphibolite Like most

monoclinic amphiboles, birefringence is in the lower second order

Cross polarized light, 100x

Q-603C

Cummingtonite in a hornblende - biotite - cummingtonite amphibolite Close-up view of

the hornblende exsolution lamellae in cummingtonite, coming out on the two irrational planes approximately parallel to {100} and {001} Some colorless cummingtonite

exsolution lamellae can be seen in hornblende to the right of the cummingtonite

Plane polarized light, 400x

Q-603C

Gedrite in a gedrite-cordierite-biotite gneiss Gedrite has colors that range from

colorless to gray to green to brown Gedrite is commonly associated with aluminous minerals like cordierite, garnet, staurolite, and aluminosilicates, as well as with other amphiboles Anthophyllite is another, less aluminous orthoamphibole, separated from gedrite by a miscibility gap defined principally by Na and Al content

Plane polarized light, 20x

IMA86-G2-1

Gedrite in a gedrite-cordierite gneiss Gedrite has lower birefringence than the

monoclinic amphiboles, typically in the upper 1st order

Cross polarized light, 20x

IMA86-G2-1

Gedrite in a gedrite-cordierite gneiss Since gedrite is orthorhombic, crystals have

extinction parallel to their length (orange crystal in the image above has been rotated 45° counterclockwise and is here at extinction)

Cross polarized light, 20x

W95

Glaucophane in a blueschist Glaucophane is characteristically pleochroic in shades of

blue and purple This glaucophane is somewhat zoned, with pale cores and darker, more

Fe3+-rich rims

Plane polarized light, 100x

C15

Glaucophane in a blueschist As with most monoclinic amphiboles, glaucophane has

birefringent colors in the lower second order

Cross polarized light, 100x

C15

Pyroxenes

Omphacite in an eclogite Omphacite is an Na-Ca-Mg-Al pyroxene, and holds the albite

component in this feldspar-free rock Omphacite is pale green and slightly pleochroic.Plane polarized light, 100x

IG16-36

Omphacite in eclogite Like most clinopyroxenes, omphacite has lower second order

birefringence

Cross polarized light, 100x

IG16-36

Sheet silicates

Talc in a soapstone (metamorphosed harzbergite) Talc is colorless and resembles

muscovite or colorless phlogopite but is much softer In hand specimen the two are easy

to tell apart: the soapy feel of talc is unlike that of the much harder micas

Plane polarized light, 40x

4.6.84A

Talc in a soapstone (metamorphosed harzbergite) The birefringence of talc is similar to

muscovite and phlogopite (colorless Mg-biotite) Several long-thin grains of low

birefringence antigorite are visible as well

Cross polarized light, 40x

4.6.84A

Talc in a soapstone (metamorphosed harzbergite) This talc crystal has been rotated to

extinction, but there are many small areas in the large central grain that are not extinct because of surface damage caused by the thin section grinding process The micas and calcite have this surface damage effect too, but not so strongly

Cross polarized light, 400x

4.6.84A

Antigorite in a soapstone (metamorphosed harzbergite) Antigorite is a serpentine

mineral that is platy, unlike fibrous asbestos It is typically colorless to pale green, and resembles chlorite

Plane polarized light, 100x

4.6.84A

Antigorite in a soapstone (metamorphosed harzbergite) Antigorite commonly has

anomalous lower first order birefringence, like chlorite Unlike chlorite, however, it can have both anomalous Berlin blue and anomalous brown interference colors in different orientations Low birefringence chlorite, in contrast, is either almost entirely anomalous blue (Fe-rich), anomalous brown (Mg-rich), or anomalous violet (intermediate

composition) The anomalous colors are caused by high dispersion of the 2V

Plane polarized light, 100x

4.6.84A

Epidotes

Epidote in an epidote-amphibolite facies basaltic rock Epidote is the term for the

optically negative, high birefringence Al-Fe3+ epidote It is probably the most common epidote type It has parallel extinction relative to its usual long crystal dimension, but inclined excinction relative to its {001} cleavage Its interference colors commonly seem anomalously bright compared to pyroxenes, possibly a result of the high dispersion of the 2V in this mineral Although commonly light pistacio yellow-green in thin section, it may be colorless Note that the term "pistacite" for Fe-rich epidotes has been discredited

by the IMA

Plane polarized light, 100x

NOR-116

Epidote, showing its interference colors along with plagioclase, quartz, hornblende, and

biotite Epidote interference colors commonly seem anomalously bright or 'fluorescent' compared to pyroxenes, possibly a result of high dispersion of the 2V in this mineral.Cross polarized light, 100x

NOR-116

Clinozoisite in an Mg-rich epidote amphibolite facies basaltic rock Although clinozoisite

is usually colorless, here it is pleochroic colorless to pale yellow Clinozoisite is optically positive, in contrast to epidote, and has lower birefringence

Plane polarized light, 100x

NOR-281

Clinozoisite , zoned with respect to Fe3+ content and birefringence The interference

colors of clinozoisite are anomalous, ranging from anomalous first order blue and brown for Fe-poor varieties, to anomalous first order lemon yellow The anomalous interferencecolors are the result of strong dispersion of the optic axes

Cross polarized light, 100x

NOR-281

Oxides

Rutile in a cordierite-gedrite gneiss Rutile is characteristically deep yellow-brown or

orange-brown in color, with enormously high relief It can accept limited amounts of uranium and thorium into its structure, and so can produce radiation halos that are generally weaker than those surrounding zircon and allanite (no halo visible here).Plane polarized light, 100x

W95

Rutile in a cordierite-gedrite gneiss Rutile has very high birefringence, rarely

discernable because of the dark mineral color and because the high colors are mostly pastels Birefringence is discernable in very small or thin crystals or fibers Typically, the pastel high order interference colors take on the yellow-brown color of the mineral.Cross polarized light, 100x

W95

Spinel in a hornblende-pyroxene granulite Spinel in common rocks is usually an

intermediate solid solution, and so is usually green It has very high relief

Plane polarized light, 100x

TMW96-A4-A

Spinel is isotropic, but its green color distinguishes it from colorless to pink garnet and

colorless fluorite, and its high relief and color from sodalite (the latter two generally occur

in different kinds of rocks, too)

Cross polarized light, 100x

TMW96-A4-A

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Kurt Hollocher Geology Department Union College Schenectady, NY 12308 U.S.A.