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Definition: syenite from The Columbia Encyclopedia

(sì'ənìt), coarse-grained igneous rock, 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.


Summary Article: Syenite
from Rock and Gem

SYENITES MAY BE VISUALLY SIMILAR to granite, and are often confused with it, but they can be distinguished from it by the absence or scarcity of quartz. A syenite is any one of a class of rocks essentially composed of an alkali feldspar or sodic plagioclase (or both), a ferromagnesian mineral – usually biotite, hornblende, or pyroxene – and little or no quartz. The alkali feldspars can include orthoclase, albite, or less commonly, microcline. Other minerals that can occur in small amounts in syenite include sphene, apatite, zircon, magnetite, and pyrite. When syenites contain quartz, they are quartz syenites. In the absence of quartz, feltspathoid minerals such as leucite, cancrinite, sodalite, or particularly nepheline, may be present.

  • Properties
  • Rock type Intermediate, plutonic, igneous
  • Major minerals Potassium feldspar
  • Minor minerals Sodium plagioclase, biotite, amphibole, pyroxene, feldspathoids
  • Color Gray, pink, or red
  • Grain size Medium to coarse

Larvikite

This iridescent variety of syenite contains feldspar, pyroxene, mica, and amphibole.

© 2008 Dorling Kindersley Limited

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