syenite

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syenite

(sī`ənīt), coarse-grained igneous rockrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
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, similar in appearance and composition to granite. Unlike granite, it contains little or no quartz. The chief minerals in syenite are the feldspars, with mica, hornblende, and pyroxene. Varieties are distinguished (according to the ferromagnesian minerals contained) as augite syenite, hornblende syenite, mica syenite, and nepheline syenite. Syenites are comparatively rare rocks, being found chiefly in a few areas of the United States and Germany. They are occasionally substituted for granites as building stones.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Syenite

 

a holocrystalline plutonic rock composed chiefly of orthoclase and plagioclase, with admixtures of the following ferrous minerals: hornblende, biotite, pyroxene, and, occasionally olivine. Quartz is also sometimes present.

Depending on the content of ferrous minerals, syenite is classified as hornblende syenite, micaceous syenite, quartz syenite, and so forth. Syenite is characterized chemically by a silica content of 55 to 65 percent; the alkali content determines the classification of syenite into normal and alkali types. In normal syenites plagioclases are present as oligoclase and andesite. Alkali syenites contain orthoclase and, more rarely, albite. Nephelines or other feldspathoids, such as leucite and the so-dalite group of minerals, are formed when the alkali content exceeds 12 percent and the silica content is reduced; alkali syenites thus grade into nepheline syenites or other feldspathoid syenites. Monzonites or gabbro syenites are similar to syenites. They are composed of labradorite and orthoclase with admixtures of ferrous minerals, such as pyroxene.

In the USSR, syenites are abundant in the southern Ukraine (Volyn’ Oblast), the Urals, Kazakhstan, the Caucasus, and Middle Asia, where monzonites are found. Syenites are used for the same purposes as granites.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

syenite

[′sī·ə‚nīt]
(petrology)
A visibly crystalline plutonic rock with granular texture composed largely of alkali feldspar, with subordinate plagioclose and mafic minerals; the intrusive equivalent of trachyte.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Overall, these rocks are much less magnetic than the syenitic units in the western part of the complex; the highest measured susceptibility is 9 x [10.sup.-3] SI units, and most samples have susceptibilities of less than 1.
Syenitic lateritic profiles of Cameroon (Braun et al., 1990) and of Brazil (Boulange and Colin, 1994) show higher positive Ce anomalies (Ce/[Ce.sup.*] = 7.01); Ce is probably adsorbed on clay fraction (Aagard, 1974).
Granosyenitic, syenitic, and quartz monzonitic rocks (mangeritic rocks after Bogatikov & Birkis, 1973) and associated gabbro-anorthosites and ultramafic rocks are found in the southern part of the batholith (Fig.
The relatively coarse grain size of xenoliths and xenocrysts indicate origin from subjacent plutonic sources, and the gabbro of Christmas Mountain and syenitic rocks of Dominguez Mountain show that such plutons exist in the Big Bend region.
Mount Kupe is a steep-sided, cone-shaped mountain of horst uplifts and syenitic and granitic intrusions formed by block faulting and bounded by structural troughs, within which volcanic activity has created small cones (Tye 1986).
Carbonatite is also spatially associated with minor syenitic, granitic, monzo-dioritic (alkaline) composition pegmatite and aplite dyke/sills.
In FPS and other skarn deposits, zeolites occur in cavities in coarse-grained calcite, in joints and fractures in calc-silicate skarn and syenitic xenoliths, or as free-standing crystals on earlier-formed crystals of meionite, diopside, feldspars, titanite, fluorapatite, and other minerals that occur in cavities resulting from the dissolution of calcite once in contact with the skarn minerals.
In order of decreasing age they are (1) agmatite containing blocks of host rock in a coarse-grained igneous matrix, (2) felsite, rhyolite porphyry, and microgranite, (3) layered and massive gabbro, (4) medium- to coarse-grained amphibolebiotite granite, (5) heterogeneous mafic rocks with a generally dioritic to tonalitic matrix containing schliers and blocks ranging from layered gabbro, to monzonitic, syenitic and granitic varieties.
Birmingham (1987) reported an occurrence of topaz in syenitic fragments in hydrothermal breccia from Globe Hill (Thompson et al., 1985; T.
Since about 1925, syenitic intrusions exposed in work around the Oberhasli and Oberaar power stations have yielded small milarite crystals, and in 1949 or 1950 a tunnel working near the Oberaar power station produced a single, sharp, chloritized crystal 3.8 cm long (Stalder, 1979).