Syenite
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 .
The
feldspar component of syenite is predominantly alkaline in character .
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 peralkaline and peraluminous, with high proportions of alakali elements and aluminium.
Encyclopedia
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 .
The
feldspar component of syenite is predominantly alkaline in character .
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 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.
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 ,
Switzerland,
Germany, and
Norway.
Etymology
The term syenite was originally applied to hornblende granite like that of
Syene in
Egypt, from which the name is derived.
Episyenite
Episyenite is a term used in petrology to describe to the result of alteration of a SiO
2 rich rock to a more SiO
2 depleted rock. The term stems from "epi" and syenite.
The process which results in SiO
2 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 SiO
2 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 SiO
2- undersaturated fluid may lead to the dissolution of
quartz from the un-altered rock, thus depleting it of this component.
See also:
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