FELSIC ROCK

igneous rock dominated by the light-coloured, silicon- and aluminum-rich minerals feldspar and quartz (qq.v.). The presence of these minerals gives felsic rock its characteristic light gray colour. The silica (SiO2) content of felsic rock is greater than about 60 percent by weight. Slight colour variations result from the presence of small amounts of mafic minerals (i.e., dark minerals rich in magnesium and iron). The colour index or volume percent of dark minerals is used as a macroscopic quantitative measure of felsic (or mafic) character. Typical felsic rocks include granite and its fine-grain extrusive equivalent, rhyolite. See also mafic rock.