Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%).

Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition.

Syenites are products of alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones.

Conversely in certain conditions, large volumes of anorthite crystals may precipitate from thoroughly molten magma in a cumulate process as it cools.

Episyenite (or epi-syenite) is a term used in petrology to describe veins, pods, or lenses of rock originally rich in silicon dioxide (SiO2) from which quartz has been severely depleted.

[11] Notable occurrences of episyenite are found in the Central Iberian Massif of Spain,[9] in Cambrian to Ordovician beds of New Mexico and Colorado,[10] Scandinavia, Brazil, and Ukraine.