Teschenite

Teschenite is a holocrystalline, inequigranular, and mesocratic intrusive igneous rock. It is a type of essexite, characterized by a mineral composition that includes plagioclase feldspar, augite, and nepheline. Olivine may also be present, and in some varieties, leucite can replace nepheline. The defining feature of teschenite is the presence of titanaugite, a variety of augite rich in titanium, which often imparts a purplish or bluish hue to the rock.

The texture of teschenite is typically medium-grained to coarse-grained, with a somewhat ophitic or subophitic arrangement where the augite crystals enclose the plagioclase laths. The dark minerals, primarily titanaugite and sometimes titanomagnetite, give it a dark color, though the presence of nepheline or leucite means it is not purely melanocratic. The rock is relatively rare and is found in specific geological settings, often associated with alkaline magmatism.

Historically, teschenite was first described from the region around Cieszyn (Teschen in German) in Silesia, hence its name. It is considered a member of the basalt-trachyte-andesite suite of rocks, specifically within the alkaline series. The geological formation of teschenite is linked to differentiation processes in silica-undersaturated magmas. Its occurrence is important for understanding the petrogenesis of alkaline intrusive rocks.