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Diorite

Pictures of Igneous Rock Types

From Andrew Alden, About.com

Diorite is a plutonic rock that is something between a granite and a gabbro. It consists mostly of white plagioclase feldspar and black hornblende. (more below)
Black and whitePhoto courtesy State of New South Wales through the Department of Education and Training
Unlike granite, diorite has no or very little quartz or alkali feldspar; unlike gabbro it is fairly light colored—half black and half white—and under the microscope it has sodic, not calcic plagioclase. If a dioritic rock is erupted from a volcano (that is, if it is extrusive), it cools into andesite lava.

The official definition of diorite is narrower than what the average geologist calls diorite. With more quartz, diorite becomes tonalite. With more alkali feldspar, diorite becomes monzonite. With more of both minerals, diorite becomes granodiorite. Maybe this will be clearer on the classification triangle.

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