SYENITE, in petrology, the group name for a class of plu tonic rocks composed essentially of an alkali felspar and a fer romagnesian mineral. A special group of alkali-syenites is char acterized by the presence of a felspathoid mineral (nepheline, leucite, analcime or sodalite).
The name was first used by Pliny for the rock occurring at Syene (Assuan) on the Nile in Upper Egypt. This rock, so ex tensively worked in ancient times for monumental structures, is a hornblende-granite with abundant quartz; Werner subsequently adopted the name for the class of rocks defined above, of which the type example is the hornblende-syenite of the Plauen'scher Grund, near Dresden. As essential constituents there occur soda orthoclase and green hornblende with subordinate amounts of oligoclase felspar. The texture of syenites is like that of the granites, hypidiomorphic-granular. Mineralogically these rocks differ from granites only by the absence or scarcity of quartz. The alkali-felspars include orthoclase, perthite or albite, more rarely microcline, while the ferromagnesian mineral may be biotite, hornblende or pyroxene. In the alkali-syenites, the am phiboles or pyroxenes are frequently soda-bearing varieties. The more normal syenites are divisible into augite-, hornblende-, and biotite-syenites according to their prevalent dark-coloured min eral, but syenites are also divisible, as in the case of granites, into potash and soda syenites, according to the type of alkali.. felspar. The accessory constituents include sphene, apatite, zir con, magnetite and pyrites. Quartz, except in the special group of alkali-syenites, is rarely absent, but never abundant. A special suite of accessory minerals, cancrinite, eudialyte eucolite, kata pleiite, melanite, corundum, etc., is found in this latter class.
Syenites are by no means common, and are not of equal im portance with granites and diorites from a geological standpoint.
Among the potash syenites, are classed the original hornblende syenite (Plauen), and the well known hornblende-syenite of Biella, Piedmont, of which analyses are given below.
The rocks known as nordmarkite, akerite, larvikite and pulas kite are classed among the soda syenites. Of these the first two are quartz-bearing and were originally described from the Oslo region (Norway). Nordmarkite is built up of pink microper thite and subordinate aegirine and arfvedsonite together with some quartz. Akerite is a quartz bearing augite-syenite with con siderable plagioclase. The larvikites are augite-syenites with large cryptoperthitic (anorthoclase) felspars possessing a re markable schiller structure. They are coarse-grained rocks, orig inally described from Larvik (Norway), and their great fresh ness and iridescent appearance when polished make them f a vourite ornamental stone for façades and pillars. The subordinate constituents are titaniferous augite, olivine, and barkevikitic hornblende. The pulaskite syenites, first described from the Fourche mountains, Ark., consist of soda-orthoclase, alkali hornblende together with some biotite. The occasional presence of nepheline indicates their close relation to the nepheline-sye nites (q.v.). Rocks transitional between syenites and diorites are known as monzonites (q.v.).
The following analyses show the chemical composition of a few of the principal types of syenite. They are characterized by a moderate amount of silica, relatively high alkalis and alumina, while lime and magnesia are more variable, but never in great amount.