colour index

colour index

In igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals in the rock. The most common light-coloured minerals are feldspars, feldspathoids, and silica or quartz; abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals.

This entry comes from Encyclopædia Britannica Concise.
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