Lattice parameter determinations of pure sodium nephelines, crystallized from gels and glasses at 1,000 Kg/cm² water vapour pressure, show that the parameters of this mineral are very variable and that they can not be correlated either with their crystallization temperatures or with the length of the experiments, unlike the parameters of albite (MacKenzie, 1957). In contrast, the lattice parameters of nephelines of composition Na₃KAl₄Si₄O₁₆, which more closely approach those of natural nephelines and of pure kalsilites, do not vary with their crystallization temperatures. Investigation of the lattice parameters of nepheline solid solutions in the systems NaAlSiO₄--NaAlSi₃O₈--H_{2O, and NaAlSiO4--CaAl2Si2O8--H2O indicates that the addition of very small amounts of both the NaAlSi3O8 and CaAl2Si2O8 molecules produces an increase in the c parameter and also stabilizes both the a and c parameters of the nepheline solid solutions. No change in nepheline solid-solution lattice parameters was found in the system Na3KAl4Si4O16--CaAl2Si2O8--H2O. These results are discussed in terms of three hypotheses proposed by Smith and Tuttle (1957), and it is concluded that the main factor causing the variability in pure nepheline parameters is the collapse of the framework structure ab ut the small sodium atom, although the substitution of hydroxyl ions for oxygen atoms and variations in the starting materials in these experimental studies may also contribute to these variations.}

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