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nepheline 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.