An important member of the feldspathoid group of silicate minerals with composition Na₃(Na,K)[Al₄Si₄O₁₆], which approximates to NaAlSiO₄ (with some K replacing Na); nepheline is an end-member of the series with kalsilite (KAlSiO₄); sp. gr. 2.56–2.66; hardness 5.5–6.0; hexagonal; colourless, grey, yellowish, brownish, reddish, or greenish; vitreous to greasy lustre; crystals normally prismatic, short, and columnar, thick, and tabular, or coarse, granular aggregates; cleavage imperfect prismatic {1010}, poor basal {0001}; it is a primary crystallizing mineral of silica-poor alkaline igneous rocks in association with aegirine augite and alkali amphiboles, and is an essential mineral in some silica undersaturated rocks (see silica saturation) such as nepheline-syenites and nephelinites.