félsica

Félsica is a category of igneous rocks defined by high silica content and a mineralogy dominated by light-colored minerals such as feldspar and quartz. The term parallels the English word felsic and contrasts with mafic rocks, which are richer in magnesium and iron.

In terms of chemistry, félsic rocks typically contain roughly 65% or more SiO2, and relatively high alkali contents, with low amounts of dark ferromagnesian minerals. The texture is usually phaneritic in plutonic varieties like granite, and aphanitic in volcanic varieties like rhyolite or dacite. The light coloration reflects the mineral assemblage rather than a lack of iron.

Formation and tectonics: félsic magmas form by partial melting of continental crust or through differentiation of mafic magmas; they crystallize at lower temperatures than mafic magmas, giving coarser plutonic textures or finer volcanic textures. They are common in continental crust and in volcanic arcs, where thick crust favors fractional crystallization.

Occurrences and uses: félsic rocks are economically important as sources of quartz and feldspar for glass and ceramics; granites are widely used in construction; some granitic intrusions host hydrothermal ore deposits.

See also: Igneous rocks; felsic vs mafic; granitoids; rhyolite.