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Definition: diorite from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Igneous rock intermediate in composition between mafic (consisting primarily of dark-coloured minerals) and felsic (consisting primarily of light-coloured minerals) – the coarse-grained plutonic equivalent of andesite. Constituent minerals include feldspar and amphibole or pyroxene with only minor amounts of quartz.


Summary Article: Diorite
from Guide to Minerals, Rocks and Fossils

Colour Speckled black and white in hand specimen; occasionally shades of dark green or pink. The dark minerals are more noticeable than in gabbro. Colour index 40 to 90, but very variable, often over short distances. Grain size Coarse, may be pegmatitic. Texture Equigranular or porphyritic. In porphyritic varieties the feldspar or hornblende may form phenocrysts. Diorites often vary rapidly in texture; an equigranular variety may grade into a porphyritic one within a few centimetres. They are sometimes foliated due to the roughly parallel arrangement of the minerals. Structure Xenoliths are common. Mineralogy Essentially plagioclase (oligoclase or andesine) and hornblende; biotite and/or pyroxene may occur. Alkali feldspar and quartz (quartz diorites) may be present, when diorite grades into granodiorite. Common accessory minerals are apatite, titanite and iron oxides. Field relations Forms independent stocks, bosses and dykes, but also comprises local variants of masses of granite, and sometimes gabbro, into which they merge imperceptibly.

© 2003 Philip's

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