Difference between revisions of "Syenite" - New World Encyclopedia

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Syenite

leucocratic variety of nepheline syenite from Sweden (särnaite).

Syenite is an uncommon, coarse-grained, intrusive igneous rock. It has the same general composition as granite, but the quartz component is either absent or present in relatively small amounts (less than five percent).

Etymology

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

Composition

Syenites are usually either peralkaline (in which the molecular proportion of aluminum is less than that of sodium oxide and potassium oxide combined) or peraluminous (in which the molecular proportion of aluminum oxide is greater than that of sodium oxide and potassium oxide combined).

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

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

Formation and occurrence

Syenites are formed during alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. A syenite is produced when a granitic or igneous rock undergoes a fairly low degree of partial melting—a condition that is required because potassium is an incompatible element and tends to enter a melt first. Higher degrees of partial melting will liberate more calcium and sodium, producing 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.

Some of the more important occurrences of syenite are in New England, Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, and Norway.

Episyenite

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

The process that 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 unaltered rock, thus depleting it of this component.

References
ISBN links support NWE through referral fees

Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W. H. Freeman. ISBN 0716724383

Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060

Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Revised ed. New York: St. Martin's Press. ISBN 1582381321

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@@ Line 1: / Line 1: @@
 {{Copyedited}}{{Claimed}}{{Contracted}}{{Images OK}}{{Submitted}}{{Approved}}{{Paid}}
 [[Image:Syenite.jpg|thumb|'''Syenite''']]
-[[Image:Nepheline-syenite-2005.jpg|thumb|[[leucocratic]] variety of [[nepheline]] syenite from [[Sweden]] ([[särnaite]])]]
+[[Image:Nepheline-syenite-2005.jpg|thumb|[[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 (less than five percent).
+'''Syenite''' is an uncommon, coarse-grained, intrusive [[igneous rock]]. It has the same general composition as [[granite]], but the [[quartz]] component is either absent or present in relatively small amounts (less than five percent).
-The [[feldspar]] component of syenite is predominantly [[alkaline]] in character (usually [[orthoclase]]). [[Plagioclase]] feldspars may be present in small quantities, less than ten percent.
+==Etymology==
+The term syenite was originally applied to hornblende granite like that obtained from [[Syene]] in [[Egypt]], from which the name is derived.
+== Composition ==
+Syenites are usually either [[peralkaline]] (in which the molecular proportion of aluminum is less than that of sodium oxide and potassium oxide combined) or [[peraluminous]] (in which the molecular proportion of aluminum oxide is greater than that of sodium oxide and potassium oxide combined).
+The [[feldspar]] component of syenite is predominantly [[alkaline]] in character (usually [[orthoclase]]). [[Plagioclase]] feldspars may be present in small quantities—less than ten percent.
 When present, ferromagnesian minerals are usually [[hornblende]] [[amphibole]], rarely [[pyroxene]] or [[biotite]]. Biotite is rare, because in a syenite [[magma]] most [[aluminum]] is used in producing feldspar.
-Syenites are usually [[peralkaline]] and [[peraluminous]], with high proportions of alkali elements and aluminum.
+== Formation and occurrence ==
-Syenites are formed from alkaline igneous activity, generally formed in thick [[continental crust]]al areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a [[granite|granitic]] or [[igneous rock|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]].
+Syenites are formed during alkaline igneous activity, generally formed in thick [[continental crust]]al areas, or in Cordilleran subduction zones. A syenite is produced when a [[granite|granitic]] or [[igneous rock]] undergoes a fairly low degree of [[partial melting]]—a condition that is required because potassium is an incompatible element and tends to enter a melt first. Higher degrees of partial melting will liberate more calcium and sodium, producing 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 [[gneiss]]es), [[Switzerland]], [[Germany]], and [[Norway]].
+Some of the more important occurrences of syenite are in [[New England]], [[Arkansas]], [[Montana]], [[New York]] (syenite [[gneiss]]es), [[Switzerland]], [[Germany]], and [[Norway]].
-===Etymology===
+==Episyenite==
-The term syenite was originally applied to hornblende granite like that of [[Syene]] in [[Egypt]], from which the name is derived.
-===Episyenite===
+'''Episyenite''' (or '''epi-syenite''') is a term used in [[petrology]] to describe the result of alteration of a SiO<sub>2</sub>-rich rock to a more SiO<sub>2</sub>-depleted rock. The term stems from "epi" and '''syenite'''.
-'''Episyenite''' (or '''epi-syenite''') is a term used in [[petrology]] to describe to the result of alteration of a [[SiO<sub>2</sub>]] rich rock to a more SiO<sub>2</sub> depleted rock. The term stems from "epi" and '''syenite'''.
-The process which results in SiO<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub>- undersaturated fluid may lead to the dissolution of [[quartz]] from the un-altered rock, thus depleting it of this component.
+The process that results in SiO<sub>2</sub> depletion can be termed ''episyenitization''. This process is only referring to the [[macroscopic]] result of relative SiO<sub>2</sub> depletion in a rock. The actual physical process leading to this SiO<sub>2</sub> 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<sub>2</sub>- undersaturated fluid, may lead to the dissolution of [[quartz]] from the unaltered rock, thus depleting it of this component.
 == See also ==
+* [[Feldspar]]
+* [[Granite]]
 * [[Mineral]]
 * [[Rock (geology)]]