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(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.



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.


A visibly crystalline plutonic rock with granular texture composed largely of alkali feldspar, with subordinate plagioclose and mafic minerals; the intrusive equivalent of trachyte.
References in periodicals archive ?
Sampling focused mainly on the episyenites and partly altered two-mica granites of the southern part of the Okrouhla Radoun uranium deposit.
In some cases, the most strongly albitized episyenites contain also microscopic druses of epitaxy-crystallized albite IV.
These later generations of chlorite are very rare in episyenites originated by alteration of two-mica granites of the Destna type due to the small amount of magmatic biotite.
Carbonates fill vugs formed during leaching of magmatic quartz and/or form lenses and veins along smaller faults in episyenites (Fig.
The origin of episyenites is, due to the dissolution of K-feldspar, connected with a depletion in rubidium and sometimes also with the evolution of a prominent negative europium anomaly (Eu/Eu* = 0.
The hydrothermal alteration of granites and the origin of episyenites in the southern part of the Okrouhla Radoun uranium deposit were accompanied by significant loss of silica, usually also by a loss of potassium.
2] is also characteristic for episyenites from the Western European uranium deposits (Cathelineau, 1987; Dubessy et al.
The origin of episyenites at the Okrouhla Radoun uranium deposit was accompanied by changes in the distribution of REE.
The origin of hydrothermal mineralization connected with the formation of episyenites in two-mica granites of the Okrouhla Radoun uranium deposit was a relatively long-lasting process.