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A light-colored, coarse-grained igneous rock consisting primarily of alkali feldspar together with some mafic minerals, especially hornblende. Unlike most igneous rocks, syenite has little or no quartz. It is believed to form from the cooling of magma that forms at very high temperatures and at great depths. It is the coarse-grained equivalent of trachyte.
any of a class of intrusive igneous rocks essentially composed of an alkali feldspar and a ferromagnesian mineral. A special group of alkali syenites is characterized by the presence of a feldspathoid mineral such as nepheline, leucite, cancrinite, or sodalite (see nepheline syenite). Chemically, syenites contain a moderate amount of silica, relatively large amounts of alkalies, and alumina. The name was first used by Pliny the Elder.