Color index (geology)

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. In the field, it can be generally estimated visually from hand specimens.

The most common light-colored (felsic) minerals are the feldspars, feldspathoids, and silica or quartz. Common dark-colored (mafic) minerals include olivine, pyroxene, amphibole, biotite, tourmaline, iron oxides, sulfides, and metals. 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.

Classifications

Melanocratic and leucocratic rocks

Based on their color index, rocks can be classified as melanocratic (higher color index), leucocratic (lower color index), or mesocratic (middle color index), as well as hololeucocratic and holomelanocratic (extremes to either side). For example, leucocratic granite would be brighter and have a lower color index than regular granite. The exact percentages separating the classes vary between petrologists.

According to Comenius University in Bratislava's Atlas of Magmatic Rocks, color indices 0–10 are hololeucocratic, 10–35 are leucocratic, 35–65 are mesocratic, 65–90 are melanocratic, and 90–100 are holomelanocratic. According to the Encyclopedia of Igneous and Metamorphic Petrology (1989), and the American Geological Institute Glossary of Geology and Related Sciences (1957), color indices 0–30 are leucocratic, 30–60 are mesocratic, and 60–100 are melanocratic. The Oxford Dictionary of Earth Sciences (2013, 4th ed.) gives a third definition in which color indices 0–5 are hololeucocratic, 5–30 are leucocratic, 30–60 are mesocratic, and 60–90 are melanocratic, not defining holomelanocratic. Some of these definitions are listed below:

Felsic and mafic rocks

Another classification of color index is into (mineralogically) felsic and mafic rocks. Rocks with low color indices are felsic, and those with higher color indices are mafic, although the exact thresholds used vary. This terminology conflicts with the definition of felsic and mafic rocks based on silica content. For example, a rock composed entirely of pyroxene would contain about 50% silica. Its silica content (by one common classification) would make it a mafic rock in chemical terms, but an ultramafic rock in mineralogical terms, because it would be entirely composed of a mafic mineral. Some examples of felsic rocks include granite and rhyolite, while examples of mafic rocks include gabbro and basalt.

According to the Encyclopædia Britannica, color indices, 0–50 are felsic, 50–90 are mafic, and 90–100 are ultramafic. An online geology textbook provides an example of the use of another classification scheme, in which color indices 0–15 are felsic, 15–45 are intermediate, 45–85 are mafic, and 85–100 are ultramafic.