SEPM Strata


*syenite*
igneous rocks metamorphic rocks minerals sedimentary rocks	Description A felsic igneous rock from the bottom of Bowen's Reaction Series. Typically light colored (few mafic minerals) with pink orthoclase being the most obvious mineral (although it can be white or greenish). quartz is sometimes present but if it is between 5 and 20% the rock is a "quartz syenite." If quartz exceeds 20% the rock shifts into the alkali granite category. Typical Minerals orthoclase - 65-100% Plagioclase - 0-35% Mafics - < 10% quartz - 0-5% Tectonic Association syenite, as with all felsic rocks, is found on continents. It is one of the last fractionation products and typically results from partial melting of the lower crust. Detail Description In this detail the pinkish colored orthoclase stands out clearly. The white translucent mineral is sodium plagioclase. There is some quartz in this specimen, most obviously the clear, glassy, grayish grain just left of center, and a similar grain on the right side of the specimen. It may be enough to call this a "quartz syenite" although this close up was taken on just about the only part of the specimen with quartz on it. The rock as a whole probably has less than 5% quartz. The black (mafic) mineral is mostly amphibole, although there may be some biotite mixed in with it. Note that the mafic minerals make up only about 5% of the rock.