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SYENITE

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Originally appearing in Volume V26, Page 280 of the 1911 Encyclopedia Britannica.
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SYENITE , a name first used by See also:

Pliny to designate rocks of the same type as the hornblendic See also:granite of Syene (Assouan) in Upper See also:Egypt, so extensively used in See also:ancient times for architertural See also:work and monuments. Transferred by See also:Werner to a See also:rock of much the same See also:appearance, though not identical in mineralogical See also:character with the See also:Egyptian granite, from the See also:Plauen 'scher Grund near See also:Dresden, it is now used as the See also:group name of a class of holo-crystalline plutonic rocks composed essentially of an See also:alkali See also:felspar and a ferromagnesian See also:mineral. The structure and appearance are very much the same as that of a hornblendic granite; from which it is difficult to distinguish these rocks in See also:hand specimens. The important difference, however, is the See also:absence or scarcity of See also:quartz in the syenites. Their essential components are See also:orthoclase, often with some See also:albite, and See also:augite, See also:hornblende or See also:biotite. The orthoclase is See also:white or See also:pink, and forms nearly one See also:half of the rock. It may be veined with albite (microperthite) and small crystals of See also:plagioclase (mostly See also:andesine and See also:oligoclase) often are See also:present, usually having better crystal-See also:line forms than the potash felspar. The prevalent hornblende is See also:green, but See also:brown hornblende and dark See also:blue hornblende, of strong pleochroism, occur in some syenites which are See also:rich in alkalis. The augite is usually See also:pale green and may be in perthitic intergrowth with the hornblende. The See also:mica is always of brown See also:colour, as See also:muscovite is not known to occur in these rocks. In the alkali syenites dark green soda augites may be present; other syenites contain a See also:violet augite which has the lamella structure of See also:diallage.

End of Article: SYENITE

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