Color index (geology)

Color index, as a geological term, is a measure of the ratio between generally dark mafic minerals and generally light felsic minerals in an igneous rock.^[1][2] The color index of an igneous rock is the volume percentage of mafic minerals in the rock, excluding minerals generally regarded as "colorless" such as apatite, muscovite, primary carbonates and similar minerals.^[1] Rocks can be sorted into classes by several systems based on their color index, including into leucocratic and melanocratic rocks, or into (mineralogically) felsic and mafic rocks.

With an accuracy within 1%, color index can be determined by applying a microscope to a flat, planar section of rock and employing a point counting technique to determine the amount of light or dark rock.^[2] In the field, it can be generally estimated visually from hand specimens.^[2]

The most common light-colored (felsic) minerals are the feldspars, feldspathoids, and silica or quartz.^[2] Common dark-colored (mafic) minerals include olivine, pyroxene, amphibole, biotite, tourmaline, iron oxides, sulfides, and metals.^[2] In their pure form, felsic minerals have a color index of 0, and mafic minerals have a color index of 100, due to being composed entirely of themselves.