Syenites are a group of plutonic igneous rocks that consist almost entirely of alkali feldspar with minor amounts of Na-rich plagioclase and that represent the intrusive counterpart of trachytes. The name ‘syenite’ derives from ‘Lapis Syenitis‘, a decorative stone that has been quarried since ancient times close to the Egyptian city of Syene (today Aswan). Alkali feldspar is the main component of these rocks, while plagioclase generally represents 10 to 30% of the feldspars and it is often present perthites (unmixings within alkali feldspar grains). The content of quartz is variable: syenites contain less than 5% of quartz, whereas quartz syenites are characterized by a quartz content between 5 and 20% of the felsic minerals. Foid-bearing syenites, the silica-undersaturated variety of these rocks, contain feldspathoids instead of quartz. ‘Foid’ in the name of these rocks can be substituted with the most abundant feldspathoid (e.g. nepheline-bearing syenite). Syenites generally contain between 10 and 35% of mafic minerals like hornblende, pyroxene, and, less commonly, biotite. Some syenites even contain Na-pyroxenes and amphiboles, like aegirine and arfvedsonite. Common accessory minerals are represented by zircon, apatite, titanite, allanite, ilmenite, and magnetite.
Syenites are uncommon rocks that are found associated with mafic rock types, like gabbros and anorthosites, evolved magmatic products, like granites, or even rarer igneous rocks, like carbonatites. The formation of syenites can be explained as a result of fractional crystallization of a parent basaltic magma and/or the assimilation of alkali-rich crustal rocks which would produce compositions enriched in alkali elements, decreasing the saturation in silica of the magma. A more in-depth discussion of the petrology of syenites is available on Alexstrekeisen.it.
Q = 0 – 20% or F < 10%
Plagioclase/feldspars = 10 – 35%
• leucosyenite (M < 10%)
• melasyenite (M > 35%)
Other varieties: quartz syenite, foid-bearing syenite
Extrusive equivalent: trachyte