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[[File:Syenite.jpg|thumb|300px|right]]'''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%). |
[[File:Syenite.jpg|thumb|300px|right]]'''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%). |
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| − | The feldspar component of syenite is predominantly alkaline in character . |
+ | The feldspar component of syenite is predominantly alkaline in character .Plagioclase feldspars may be present in small quantities, less than 10%. |
| + | ==Characteristics== |
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| − | Plagioclase feldspars may be present in small quantities, less than 10%. |
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When present, the ferromagnesian mineral is usually [[hornblende]], in a syenite magma most [[aluminum|aluminium]] is used in producing feldspars. |
When present, the ferromagnesian mineral is usually [[hornblende]], in a syenite magma most [[aluminum|aluminium]] is used in producing feldspars. |
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Revision as of 13:14, 30 October 2010
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 .Plagioclase feldspars may be present in small quantities, less than 10%.
Characteristics
When present, the ferromagnesian mineral is usually hornblende, in a syenite magma most aluminium is used in producing feldspars.
Syenites are usually peralkaline and peraluminous, with high proportions of alkali 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.