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What is diorite?

An intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. Similar to gabbro only not as so dark, and containing less iron and magnesium.

'Salt and Pepper' Rock:
Diorite is of intermediate composition. It is called 'salt and pepper' rock because of its nearly equal proportions of light, felsic minerals (mostly plagioclase and quartz) and dark, mafic minerals (amphibole and biotite). Diorite is close in composition to granodiorite, and it requires fairly close inspection to differentiate the two rocks. Diorite typically contains no orthoclase feldspar (pink potassium feldspar).


Learn more:
USGS/NPS Geology in the Parks Website, 2003, and Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195


Tags: Minerals, Rocks