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 (<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 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, 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, 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" (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.

ee also

* list of minerals
* list of rock types


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  • syénite — [ sjenit ] n. f. • 1765; siémite 1611; lat. syenites, gr. suênitês « de Syène », a. nom d Assouan ♦ Minér. Roche plutonique grenue, composée essentiellement de feldspath alcalin, d un peu de biotite et de hornblende, de couleur blanche, rosée ou… …   Encyclopédie Universelle

  • Syenite — Syénite La syénite est une roche magmatique plutonique grenue de couleur rose à rouge. Elle est composée de feldspath alcalin, de biotite et de hornblende. Son nom vient de Syène, ancien nom de la ville d Assouan (Égypte). On peut y ajouter, que… …   Wikipédia en Français

  • Syenite — Sy e*nite, n. [L. Syenites (sc. lapis), from Syene, Gr. ?.] (Min.) (a) Orig., a rock composed of quartz, hornblende, and feldspar, anciently quarried at Syene, in Upper Egypt, and now called {granite}. (b) A granular, crystalline, ingeous rock… …   The Collaborative International Dictionary of English

  • syenite — [sī′ə nīt΄] n. [Fr syénite < L Syenites (lapis), Syenite (stone) < Syene < Gr Syēnē, Syene] a grayish, intrusive igneous rock usually containing feldspar, hornblende, and some quartz syenitic [sī΄ənit′ik] adj …   English World dictionary

  • Syénite — La syénite est une roche magmatique plutonique grenue de couleur rose à rouge. Elle est composée de feldspath alcalin, de biotite et de hornblende. Son nom vient de Syène, ancien nom de la ville d Assouan (Égypte). On peut y ajouter, que l… …   Wikipédia en Français

  • syenite — syenitic /suy euh nit ik/, adj. /suy euh nuyt /, n. a granular igneous rock consisting chiefly of orthoclase and oligoclase with hornblende, biotite, or augite. [1790 1800; < L syenites (lapis) (stone) of SYENE < Gk syenítes (líthos); see ITE1] * …   Universalium

  • syenite — [ sʌɪənʌɪt] noun Geology a coarse grained grey igneous rock composed typically of alkali feldspar and hornblende. Derivatives syenitic adjective Origin C18: from Fr. syénite, from L. Syenites (lapis) (stone) of Syene , Greek name of Aswan in… …   English new terms dictionary

  • syenite — sienitas statusas T sritis chemija apibrėžtis Vidutinio rūgštingumo magminė intruzinė uoliena. atitikmenys: angl. syenite rus. сиенит …   Chemijos terminų aiškinamasis žodynas

  • syenite — noun Etymology: Latin Syenites (lapis) stone of Syene, from Syene, ancient city in Egyptian Date: circa 1796 an igneous rock composed chiefly of feldspar • syenitic adjective …   New Collegiate Dictionary

  • syenite — noun An igneous rock composed of feldspar and hornblende …   Wiktionary

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