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Granodiorite

Pictures of Igneous Rock Types

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Granodiorite is a plutonic rock composed of black biotite, dark-gray hornblende, off-white plagioclase, and translucent gray quartz. (more below)
An in-between rock type

Click the photo for a larger version

Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
Granodiorite differs from diorite by the presence of quartz, and the predominance of plagioclase over alkali feldspar distinguishes it from granite. Although it isn't true granite, granodiorite is one of the granitoid rocks. Rusty colors reflect weathering of rare grains of pyrite, which releases iron. The random orientation of grains shows that this is a plutonic rock.

This specimen is from southeastern New Hampshire. Click the photo for a larger version.

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