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Gabbro: Gabbro is coarse-grained intrusive igneous rock, made of calcium-rich plagioclase, with amphibole and/or pyroxene. Composition: Gabbro is typically dark gray, greenish gray, to black, mafic in composition, with visible crystals of plagioclase feldspar (Ca-rich varieties), hornblende, pyroxene, and lesser amounts of biotite mica, magnetite, and other minerals. Gabbro is chemically equivalent to basalt. -- Excerpts from: USGS/NPS Geology in the Parks Website, 2001, and Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195 |