Syenite

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Syenite
leucocratic variety of nepheline syenite from Sweden (särnaite)

Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%).

The feldspar component of syenite is predominantly alkaline in character (usually orthoclase) . Plagioclase feldspars may be present in small quantities, less than 10%.

When present, ferromagnesian minerals are usually hornblende amphibole, rarely pyroxene or biotite. Biotite is rare, because in a syenite magma most aluminium is used in producing feldspar.

Syenites are usually either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali elements (K, Na, Ca).

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.

At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime.

Syenite is not a common rock, some of the more important occurrences being in New England, Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, Norway, Plovdiv, Bulgaria, Malawi (Mulanje Mountain Forest Reserve) and Romania (Ditrău). The Malvern Hills, which are on the border between the English counties of Herefordshire and Worcestershire are also formed from syenite.

Etymology[edit]

The term syenite was originally applied to hornblende granite like that of Syene in Egypt, from which the name is derived.

Episyenite[edit]

Episyenite (or epi-syenite) is a term used in petrology to describe to the result of alteration of a SiO2 rich rock to a more SiO2 depleted rock.

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.

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