Medium- to coarse-grained igneous rock, a member of the alkali-syenite group, which consists largely of feldspar and nepheline. Nepheline syenite from Canada is used to replace feldspar in the manufacture of ceramic and glass products.
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Syenites are formed from alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith to a fairly low degree of partial melting. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite, adamellite or tonalite.
The process which results in SiO2 depletion can be termed episyenitization. This process is only referring to the macroscopic result of relative SiO2 depletion in a rock. The actual physical process leading to this SiO2 depletion may vary in a given metamorphic environment. Diffusion of chemical components in a stagnant fluid, related to differences in chemical potential or pressure as well as advection of a SiO2- undersaturated fluid may lead to the dissolution of quartz from the un-altered rock, thus depleting it of this component.