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ERTH 2104 Study Guide - Final Guide: Iddingsite, Pleochroism, Birefringence

Earth Sciences
Course Code
ERTH 2104
Brian Cousens
Study Guide

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Igneous Pre-Lab
Olivine: green, glassy, no cleavage
-cracked, brownish-red, iddingsite alteration, clear pebbly look in thin
section, pointed terminations
-skeletal structure
-round, curved cracks at either end
-cracks between iddingsite
Pyroxene: square, green to brown, 2 good cleavages but usually poor
-square, slight green colour, no pebbly look, may see cleavage
(orthopyroxene: clear)
Amphibole: black, usually elongate in shape
-pleochroic( brown/green), excellent cleavage, 120 cross sections
-commonly rimmed by/converted to ‘opacite’ where amphibole has
- water in structure
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,
- Pleochroic, birds eye texture, bent
K-feldspar: pink, grey, greenish, Carlsbad twin (1 side black/1 white), perthitic
-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
-little tiny dotes
Apatite: green, six sided
-very low birefringence, elongate shape, commonly look like shadow
Zircon: red-brown, wedge shape
-high relief, radiation damage (dark edge) around crystals, high
birefringence that is commonly zoned
-older rock has higher radiation – halo=zircon
Garnet: in Al-rich granitoids with muscovite mica or eclogites with pyroxene-
melted from sedimentary, 2 micas= garnet, brownish red colour
- Isotropicm sudheral to euhedral; high refractive index of this mineral
causes fracture & the grain margin to stand out as dark lines b/c of
total internal re6ection
- Found basaltic rocks