Syenite

Syenite is a coarse-grained intrusive igneous rock that is similar to granite but contains little or no quartz. The dominant mineral is alkali feldspar—potassium-rich and/or sodium-rich varieties such as orthoclase or microcline—often with lesser plagioclase. Accessory minerals may include biotite, hornblende, or pyroxene. A notable variety is nepheline syenite, in which feldspathoids such as nepheline replace some quartz and feldspar components.

Because it crystallizes underground, syenite is typically phaneritic and coarse-grained, reflecting slow cooling in plutonic settings. The silica content is low enough that quartz is scarce or absent in most varieties; quartz-bearing varieties exist but are relatively rare and are sometimes called quartz syenite.

Geologically, syenite forms from silica-undersaturated, alkali-rich magmas in the continental crust and is commonly associated with alkaline or peralkaline complexes. It occurs as plutons, stocks, or dikes and is often part of ancient shield terrains and alkaline provinces. Nepheline-bearing syenites are closely linked to related rock types such as nepheline syenite–carbonatite complexes.

Economic significance arises from its feldspar content, which is used in ceramics and glass. Nepheline syenite, in particular, is mined as a source of alumina and as a flux in industrial glass and ceramic applications.

Syenite is distinguished from granite primarily by its near-absence of quartz; quartz-bearing varieties exist but remain a minority. The rock may be classified as quartz syenite when quartz is present, or as nepheline syenite in silica-undersaturated varieties.