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Syenites are usually peralkaline and peraluminous, with high proportions of alakali elements and aluminium.
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.
Syenite is not a common rock, some of the more important occurrences being in New England, Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, and Norway.
The term syenite was originally applied to hornblende granite like that of Syene in Egypt, from which the name is derived.
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 term stems from "epi" and syenite.
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Syenite". A list of authors is available in Wikipedia.|