ERTH 2104 Study Guide - Final Guide: Iddingsite, Pleochroism, Birefringence
17 Jan 2016
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Olivine: green, glassy, no cleavage section, pointed terminations. Cracked, brownish-red, iddingsite alteration, clear pebbly look in thin. Pyroxene: square, green to brown, 2 good cleavages but usually poor expressed. Square, slight green colour, no pebbly look, may see cleavage (orthopyroxene: clear) Commonly rimmed by/converted to opacite" where amphibole has. Plagioclase: white-grey, rarely blue(labordorite), polysynthetic twinning on one cleavage plane. Twinning, low birefringence, usually elongate, zoning, sieve texture (lots of holes- new feldspar created around as it changes) Biotite: grey-black, thin, mica cleavage, lustre of cleavage plane, K-feldspar: pink, grey, greenish, carlsbad twin (1 side black/1 white), perthitic texture birefringence, Carlsbad twins, crosshatch twinning in plutonic varieties, low commonly turbid. Quartz: clear to grey, glassy, commonly with biotite in plutonic rock b/c formed together @ same time. Clear, high relief, embayed, lots of tiny inclusions. Very low birefringence, elongate shape, commonly look like shadow. Zircon: red-brown, wedge shape birefringence that is commonly zoned.
