Granodiorite

Written by Boss Tambang Tuesday, 01 December 2009 23:02

Gallery - Rock Gallery

This image, magnified about 2.5 times, shows the typical salt-and-pepper appearance of granodiorite. The black  minerals are biotite (black mica) and dark-gray hornblende, and the light-colored minerals are yellowish-white feldspar (plagioclase and potassium feldspar) and translucent gray quartz. The quartz is what makes it granodiorite rather than diorite. Rusty colors reflect weathering of rare grains of pyrite, which releases iron. The random orientation of grains suggests that this is a plutonic, not metamorphic rock. This polished slab is from southeastern New Hampshire.

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