syenite

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syenite A saturated, coarse-grained, igneous rock consisting of essential alkali feldspar and ferromagnesian minerals (biotite, hornblende, arfvedsonite, aegirine-augite, and/or aegirine) and accessory apatite, zircon, and iron oxides. The feldspar constitutes more than 65% of the rock. Hypersolvus syenites are characterized by one type of alkali feldspar, usually potassium-rich and displaying perthitic (see PERTHITE) texture. Subsolvus syenites are characterized by two types of alkali feldspar, a potassium-rich type displaying perthitic texture and a sodium-rich type displaying antiperthitic texture. Syenites, which are the plutonic equivalents of trachytes, are found as ring complexes and as discrete intrusions on the stable continental crust and in the cores of some offaxis, ocean-island volcanoes.

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syenite (sī´ənīt), 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.

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syenite (min.) crystalline rock allied to granite. XVIII. — F. syénite, G. syenit — L. syēnītēs, f. Syēnē, Gr. Suḗnē town of Upper Egypt (now Aswan); see -ITE.