SYENITES MAY BE VISUALLY SIMILAR to granite, and are often confused with it, but they can be distinguished from it by the absence or scarcity of quartz. A syenite is any one of a class of rocks essentially composed of an alkali feldspar or sodic plagioclase (or both), a ferromagnesian mineral – usually biotite, hornblende, or pyroxene – and little or no quartz. The alkali feldspars can include orthoclase, albite, or less commonly, microcline. Other minerals that can occur in small amounts in syenite include sphene, apatite, zircon, magnetite, and pyrite. When syenites contain quartz, they are quartz syenites. In the absence of quartz, feltspathoid minerals such as leucite, cancrinite, sodalite, or particularly nepheline, may be present.
- Rock type Intermediate, plutonic, igneous
- Major minerals Potassium feldspar
- Minor minerals Sodium plagioclase, biotite, amphibole, pyroxene, feldspathoids
- Color Gray, pink, or red
- Grain size Medium to coarse
Colour Red, pink, grey or white. Colour index 0 to 40 Grain size Coarse; can be pegmatitic. Texture Tends to be equigranular, but also...
(sī'Әnīt), coarse-grained igneous rock, similar in appearance and composition to granite. Unlike granite, it contains little or no quartz. The chief
Colour Usually grey but tones of green, pink and yellow. Colour index Usually 0 to 30; but occasionally higher. Grain size Coarse; can be...