Color index (geology)

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). The Oxford Dictionary of Earth Sciences (2013, 4 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, of felsic and mafic rocks based on silica content. == Characteristics ==

Speaking broadly, mineral color points out the specific gravity of the mineral, as minerals that are lighter in color tend to be less dense. Darker minerals typically tend to contain more of relatively heavy elements, notably iron, magnesium, and calcium. ==References==

• britannica.com reference • Another link