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Diorite

Pictures of Igneous Rock Types

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Diorite is a plutonic rock that is something between granite and gabbro. It consists mostly of white plagioclase feldspar and black hornblende. (more below)
Black and white
Photo 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, diorite contains sodic, not calcic plagioclase. Typically, sodic plagioclase is the bright white variety albite, giving diorite a high-relief look. If a dioritic rock is erupted from a volcano (that is, if it is extrusive), it cools into andesite lava.

In the field, geologists may call a black-and-white rock diorite, but true diorite is not very common. With a little quartz, diorite becomes quartz diorite, and with more quartz it 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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