Blencoe, James G. Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
Last reviewed:December 2019
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A mineral of variable composition: in its purest state, NaAlSiO4; often nearly Na3K(AlSiO4)4; but generally (Na,K,□,Ca,Mg,Fe2+,Mn,Ti)8(Al,Si,Fe3+)16O32, where □ represents vacant crystallographic sites and Ca, Mg, Fe2+, Mn, Ti and Fe3+ are usually present in only minor or trace amounts. The most important variations in nepheline composition are due to crystalline solution of KAlSiO4 (the mineral kalsilite) and substitution of □ for K. Crystalline solution of KAlSiO4 in (Na,K)AlSiO4 nepheline is limited to about 25 mole % at 25°C (77°F), but increases with increasing temperature (T) to 65–70 mole % at 1000°C (1832°F). Substitution of □ for K can be viewed and treated as crystalline solution of alkali feldspar or (alternatively) “excess silica.” The amount of “excess silica” that can be present stably in any nepheline is very small at T < 500°C (932°F) and only a few mole % can be dissolved in K-rich nephelines at any temperature. However, crystalline solution of “excess silica” increases steadily in Na-rich nephelines at T > 500°C (932°F) and at 1000°C (1832°F) as much as 30 mole % can be dissolved in K-free nepheline. See also: Silicate minerals
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