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a mineral of the tectosilicate class with chemical composition KNa3 [AlSiO4]4. It usually contains an excess of Si and admixtures of Ca, Mg, and, more rarely, Fe, Be, Cl, H2O, and Ga. The structure of nepheline is based on a slightly distorted structure of tridymite, whose large holes (spaces) contain alkaline cations. The arrangement of the (Si, Al) tetrahedrons distorts the structure in such a way that it has no plane of symmetry.
Nepheline crystallizes in the hexagonal system to form massive grainy aggregates and, more rarely, small prismatic crystals. It has a hardness of 5.5–6 on Mohs’ scale and a density of 2,550–2,650 kg/m3. The cleavage is imperfect; the luster varies from vitreous to greasy. The mineral is usually colorless, pink, gray, or greenish.
Nepheline is readily soluble in acid, yielding flocculent silica. It is a principal mineral constituent of alkali rocks (nepheline syenites and their pegmatites). Nepheline alters to sodalite, cancrinite, and zeolites upon the action of postmagmatic solutions. Under supergenic conditions, it converts to hydromica, mont-morillonite, and halloysite; lateritic weathering causes nepheline to alter to gibbsite.
In the USSR, major nepheline deposits are located on the Kola Peninsula, in the Il’men’ Mountains (Urals), in Krasnoiarsk Krai, and in the Alai Range. Other deposits are found in Greenland, Norway, Sweden, the Federal Republic of Germany, Kenya, and elsewhere. In the USSR, a process has been developed permitting an efficient utilization of the mineral whereby aluminum, soda, and other products are obtained. Nepheline is also used in glassmaking, in the preparation of silica gel, and in agriculture as a fertilizer for acidic soils.
REFERENCESBetekhtin, A. G. Kurs mineralogii, 2nd ed. Moscow, 1956.
kostov, I. Mineralogiia. Moscow, 1971. (Translated from English.)
M. D. DORFMAN