Leucite in rock matrix. Polished slab of leucitite, a general name for volcanic rocks (grey) containing the mineral leucite (white). The leucite is in the form of large crystals (phenocrysts), though some of the ones seen here are a different mineral called augite. The volcanic rock consists of a mixture of mineral components like augite, titanite and magnetite. This sample is 8.5 centimetres across.