Metamorphic rocks in thin section

Mostly those not seen previously for igneous rocks

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 from the 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 diamond-shaped cross sections.

 

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