diorite

Diorite is a coarse-grained, intrusive igneous rock of intermediate composition. It crystallizes from silica- and alkali-rich magma that cools slowly below the Earth's surface, yielding an interlocking array of crystals. It is the intrusive equivalent of andesite and is typically named for its mineral assemblage rather than color.

The mineral content is dominated by plagioclase feldspar (often sodium- or calcium-rich varieties), with lesser amounts of dark minerals such as hornblende, augite (pyroxene), and biotite. Quartz is uncommon in diorite, though trace amounts may occur. The texture is phaneritic, meaning the individual minerals are large enough to be seen with the naked eye; diorites can be equigranular or occasionally porphyritic with larger phenocrysts. The rock’s appearance is commonly described as salt-and-pepper, resulting from the intergrowth of light-colored plagioclase with dark mafic minerals.

Geologically, diorite forms in magma chambers within crustal plutons and batholiths. It is considered the intermediate counterpart to granites (felsic) and gabbro/basalt (mafic). The rock is widely distributed but less abundant than granite in continental crusts, and it often occurs together with granitoids in mountain belts and continental arcs.

Uses and notable properties: Diorite has been used as a decorative and building stone since antiquity, valued for its strength and coarse texture. It is also studied for understanding crustal differentiation and magmatic processes.