Geologists sometimes find it useful to classify igneous rocks based on color. Because color is sensitive to minor chemical differences it is not a very reliable index to the history or composition of any given rock ; however, it has the merit of being obvious at a glance, making color classification an indispensable aid to describing rocks in the field. Minerals are classed in two general color groups: felsic (light) and mafic (dark). Rocks may contain a mixture of mafic and felsic minerals, and are termed felsic if felsic minerals predominate, mafic otherwise. Alternatively, a numerical color index can be assigned to a rock based on visual estimation of the percentage of mafic or felsic minerals it contains.
Felsic minerals are usually higher in silica (SO2) and aluminum and of lower density than mafic minerals. Common felsic minerals are quartz , feldspar , and the feldspathoids, and common felsic rocks (i.e., rocks high in felsic minerals) are granite and rhyolite . Mafic minerals are usually higher high in iron and magnesium than felsic minerals; common mafic minerals are pyroxene, amphibole, olivine , mica, and biotite, and common mafic rocks are basalt and gabbro.
The term mafic is also used in a precise chemical sense, that is, to denote rocks consisting of 45–52% silica regardless of color. Since the non-silica fraction of a rock often consists largely of iron and magnesium compounds, rocks that are mafic in the chemical sense are usually also mafic in the color sense.
See also Silicic