Syenogranite

Syenogranite is a fine to coarse grained intrusive igneous rock of the same general composition as granite. They are characteristically felsic.

The feldspar component of syenogranite is predominantly alkaline in character (usually orthoclase). For example, the syenogranite in the Salmon Mountains in Idaho is pink to tan and composed of 45–55% alkali feldspar, 15–20% plagioclase, 15–20% quartz, 5–8% biotite, 3–5% hornblende, and accessory magnetite (Evans and Green, 2003).

Syenogranite is similar to syenite, however the major difference is its higher content of quartz (15-25%) usually at the expense of some alkali feldspar (45-50%), the domination of biotite over hornblende and the presence of muscovite and rutile as additional accessories in the syenogranite. Some syenogranite contain rare idiomorphic amphibole (Fe-hornblende and Fe-edenite), biotite (annite 25%-35%), plagioclase (An3), along with K-feldspar and quartz. (Mafti, 2001).

In the Sao Jose do Campestre Massif in Brazil, the syenogranite is a coarse-grained rock composed of microcline, quartz, hornblende, and rare clinopyroxene, together with allanite, as essential minerals (Benjamin et al., 1998).