granite

granite coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present. Depending on the feldspar present, granite may be pink, dark gray, or light gray. It is commonly believed to have solidified from molten rock (called magma) under pressure. However, some granites show no contacts with surrounding wall rock, but instead gradually grade into metamorphic rock. Others show relic features found in sediments. This evidence suggests that some granites are not igneous in origin, but metamorphic. Some granites are the oldest known rocks on earth; others were formed during younger geologic periods. Crystallized at depth, granite masses are exposed at the earth's surface by crustal movement or by the erosion of overlying rocks. Very coarse-grained granite, called pegmatite, may contain minerals and gemstones of economic value. Such pegmatites are found in the Black Hills of South Dakota. Granite has been used since ancient times as a building material.

granite Granites are intrusive igneous rocks that contain large amounts of silica (SiO₂). They contain minerals such as feldspar, which is white or pink, quartz, which is generally colourless, and either biotite, a flaky black mineral, amphibole, a black mineral, or both.

Granitic magma is thought to be generated in two ways: either by the melting of an existing rock or by the differentiation of a basaltic magma by fractional crystallization. In the process of fractional crystallization, denser magnesium-rich and silica-poor minerals crystallize and are separated from a melt, leaving it richer in silica. If a large volume of crystals is lost, the composition of the melt will be sufficiently modified to form a granite.

Perhaps the most spectacular granite bodies are those of the cordilleras of North and South America: a series of batholiths extending from the Coast Range in Canada to Chile. Some of these plutons are of vast extent; the coastal batholith of Peru, for example, is more than 2400 km long and up to 65 km wide.

Granites are not only strong and durable but also decorative; they are thus popular as facing stone for buildings, or as restaurant counters. They are particularly popular for the decoration of banks that wish to suggest financial durability. One of the most popular British granites for such purposes is the Shap granite, quarried on the Shap Fells in Cumbria, which is easily recognized by the large (2–3 cm) pink feldspar crystals in it.

Judith M. Bunbury

granite A light-coloured, coarse-grained, igneous rock, consisting of essential quartz (at least 20%), alkali feldspar, mica (biotite and/or muscovite), with or more commonly without amphibole, and accessory apatite, magnetite, and sphene. Hypersolvus granites are characterized by one type of alkali feldspar, usually microperthite, whereas subsolvus granites are characterized by two types of alkali feldspar: microperthite and albite. Granite can be formed by partial melting of old continental crust, on a local scale by in situ replacement of continental crust (granitization), by fractional crystallization of basalt magma, or by a combination of these processes.