Metamorphic rocks in thin section

[...]... 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... 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... either of these other two minerals Plane polarized light, 40x WE-1 Cordierite in a garnet-cordierite-biotite schist Birefringence is up to 1 st 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... 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... 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... 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... 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. .. 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... 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,... 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 . METAMORPHIC ROCK IN THIN SECTION Contents Image scales (field width) Al-silicates Other aluminous minerals Amphiboles Pyroxenes Sheet silicates Epidotes Oxides 20x. 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,. 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