Metamorphic rocks in thin section

Contents Image scales (widest frame width)

Al-silicatesOther aluminous minerals

AmphibolesPyroxenes

Sheet silicatesOther metamorphic minerals

20x = 6 mm40x = 3 mm

100x = 1 mm200x = 0.5 mm

400x = 0.25 mm1000x = 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 from the corners of the diamond.  This is known a "chiastolite cross".  Note the small garnet just to the below the diamond.  Two cleavages intersecting at approximately right angles are visible.

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.

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 (var. 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 (var. 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 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 diamond-shaped cross sections..

Plane polarized light, 40x

WE-1

Sillimanite 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 is surrounded by a coarse muscovite rim (lower right) but is partially transformed also to sillimanite.

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 after 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, and has a characteristic pale to darker golden yellow pleochroism.  This example has a twin.

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

Staurolite in a muscovite-biotite schist. Closeup view of the end of a staurolite crystal, with garnet in the upper left (slightly higher relief) and muscovite in the upper right (lower relief).

Cross polarized light, 100x

Gassetts Schist

 

Staurolite in a muscovite-biotite schist. Closeup view of the end of a staurolite crystal.

Cross polarized light, 100x

Gassetts Schist

 

Chloritoid in a muscovite-chlorite phyllite.  The pale blue to green pleochroism of the chloritoid, and its high relief, contrasts sharply with the lower relief, pale yellow to green pleochroic chlorite.  This grain has inclusion sector zoning.

Plane polarized light, 20x

IMA96-C4B

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 of chloritoid is visible.

Cross polarized light, 20x

IMA96-C4B

Chloritoid in a muscovite-chlorite phyllite.  Closeup showing the green chlorite rim surrounding the bluish and much higher relief chloritoid.

Cross polarized light, 100x

IMA96-C4B

 

Chloritoid in a muscovite-chlorite phyllite.  The chloritoid also has much higher birefringence than the chlorite..

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 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 the hornblende above the cummingtonite.

Plane polarized light, 200x

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 zoned, with pale cores and darker, more Fe3+-rich rims.

Plane polarized light, 100x.

IG16-36

Glaucophane in a blueschist.  As with most monoclinic amphiboles, glaucophane has birefringent colors in the lower second order.

Cross polarized light, 100x.

IG16-36

Pyroxenes

Omphacite in a glaucophane schist.  Omphacite is an Na-Ca-Mg-Al pyroxene, and is the location of some of the albite component in this feldspar-free rock. The omphacite is pale green and non-pleochroic.

Plane polarized light, 100x

IG16-36

Omphacite in a glaucophane schist.  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 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, 200x.

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

Other metamorphic minerals

Rutile in a cordierite-gedrite gneiss. Rutile is characteristically deep yellow-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, allanite, and titanite (no halo visible here).

Plane polarized light, 100x.

W95

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

Return to the Petrology page

Union College Geology Department

Union College home page