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