A saturated, coarse-grained, igneous rock consisting of essential alkali feldspar and ferromagnesian minerals (biotite, hornblende, arfvedsonite, aegirine-augite, and/or aegirine) and accessory apatite, zircon, and iron oxides. The feldspar constitutes more than 65% of the rock. Hypersolvus syenites are characterized by one type of alkali feldspar, usually potassium-rich and displaying perthitic (see perthite) texture. Subsolvus syenites are characterized by two types of alkali feldspar, a potassium-rich type displaying perthitic texture and a sodium-rich type displaying antiperthitic texture. Syenites, which are the plutonic equivalents of trachytes, are found as ring complexes and as discrete intrusions on the stable continental crust and in the cores of some off-axis, ocean-island volcanoes.
Subjects: Earth Sciences and Geography.