Point Group
ditrigonal-scalenohedral - 3m
Habit
blocky, dipyramidal, prismatic, columnar, Scalenohedron, tabular, barrel-shaped
Formation
(1) Magmatic, through crystallization from SiO2-undersaturated melts (such as syenites or nepheline syenites) and as an accessory in Al-rich plutonites.
(2) Metamorphic, through contact or regional metamorphism of Al-rich protoliths (pelites, bauxites) under amphibolite to granulite facies P-T conditions.
(3) Metasomatic, through desilicification of aluminum-bearing rocks in contact with silica-poor intrusions or fluids.
Twinning
Contact twins after {102}
