Pronunciation: DIE•oh•right
Description*: Diorite is a phaneritic intrusive igneous rock composed primarily of intermediate-composition plagioclase feldspar (which is commonly white), amphibole and various minor minerals such as pyroxene, biotite, a white alkalai feldspar, and perhaps a very smal l amount of quartz. Diorites and andesites (their extrusive equivalent) commonly have a grayish to salt-and-pepper appearance. Lighter-toned diorites may be mistaken for granite; however, granite commonly has pink potassium feldspar and always has a su bstantial amount of quartz. Darker diorites may be mistaken for gabbro, but diorites are associated with granites while gabbros are associated with basalts. The feldspar in diorite is generally white, while the calcium-rich plagioclase in gabbro is gray . The dark minerals in diorite are primarily biotite and amphibole, while in gabbro the dark minerals are dominantly pyroxene and olivine and dark Ca-plagioclase. Diorite forms in continental crust above subduction zones, usually along with granite.
*Description from Cronin, V.S., 2001, Geology laboratory projects for group learning: Primis McGraw-Hill, 156 pp., ISBN 0-07-252348-4
Used by permission of the author.
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