Color index (geology)

[2] Common dark-colored (mafic) minerals include olivine, pyroxene, amphibole, biotite, tourmaline, iron oxides, sulfides, and metals.

[1] 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.

This terminology conflicts with the definition[7][8] of felsic and mafic rocks based on silica content.

[6] 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.

Darker minerals typically tend to contain more of relatively heavy elements, notably iron, magnesium, and calcium.