syenite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An igneous rock composed primarily of alkali feldspar together with other minerals, such as hornblende.

Century Dictionary and Cyclopedia

1. n. A rock composed of feldspar and hornblende, with or without quartz. The name syenites was given by Pliny to the red granitoid rock extensively quarried at Syene in Egypt. The term syenite was introduced into modem geological science by Werner, in 1788, bur. applied by him to a rock (from the Plauenscher Grund, near Dresden) not identical in composition with the syenites of Pliny, which latter is a hornblendic granite, or granite in which mica is replaced by hornblende, whereas the rock which Werner called syenite is mainly made up of a mixture of feldspar and hornblende; hence there has long been more or less confusion in regard to the nomenclature of this rock. The English and some continental geologists have defined syenite as an aggregate of quartz, feldspar, and hornblende; while the Germans have generally regarded the quartz as not being an essential constituent of the rock: this latter view is that which has been adopted in the most recent English geological and lithological works. Syenite is a rock thoronghly crystalline in texture, and in general it much resembles granite in its mode of occnrrence. The feldspathic ingredient is chiefly orthoclase, and this usually predominates considerably in quantity over the associated minerals; there is some triclinic feldspar present, however, in most syenites, and the same is true in regard to quartz, biotite, titanite, magnetite, apatite, zircon, and various other accessory minerals frequently found in small quantity in the granitic rocks. Sometimes the hornblende is replaced by augite; this variety is designated augite-syenite; that in which mica predominates is known as mica-syenite or minette. The range of syenite in geological age is similar to that of granite, and the frequent passage of one rock into the other shows how closely allied the two are, one result of which condition is that the nomenclature of the different varieties is correspondingly difficult. Typical syenite is by no means abundant, and in general the granitic rocks very considerably surpass the syenitic in economic importance. Also sienite.
2. n. In the quantitative system of classification (see rock), a proposed general field-term for a phaneric igneous rock composed of dominant feldspar of any kind, with subordinate amounts of mica, hornblende, pyroxene, or other minerals, and without a noticeable amount of quartz

Wiktionary

1. n. geology, obsolete granite
2. n. geology An igneous rock composed of feldspar and hornblende

GNU Webster's 1913

1. n. Orig., a rock composed of quartz, hornblende, and feldspar, anciently quarried at Syene, in Upper Egypt, and now called granite.
2. n. A granular, crystalline, ingeous rock composed of orthoclase and hornblende, the latter often replaced or accompanied by pyroxene or mica. Syenite sometimes contains nephelite (elæolite) or leucite, and is then called nephelite (elæolite) syenite or leucite syenite.