Monoclinic and Hexagonal Nepheline Structures of (Na_3/4/K_1/4)AlGeO₄

Hexagonal (Na_3/4K_1/4)AlGeO₄ crystallizes in the space group P6₃ with unit-cell parameters a = 10.164 (2), c = 8.540 (2) Å and Z = 8 [wR(F²) = 0.066 for all 3060 independent reflections]. Monoclinic (Na_3/4K_1/4)­AlGeO₄ crystallizes in the space group P2₁ with unit-cell parameters a = 10.0477 (4), b = 8.5764 (4), c = 10.2118 (4) Å, β = 119.035 (1)° and Z = 8 [wR(F²) = 0.120 for all 3194 independent reflections measured on a twinned crystal]. Both structures belong to the large family of stuffed tridymites, with the Al and Ge atoms occupying tetrahedral sites, and the alkali atoms occupying the cavities of the tetrahedral framework. Hexagonal (Na_3/4K_1/4)AlGeO₄ is isostructural with the silicate mineral nepheline (Na_3/4K_1/4)AlSiO₄, while monoclinic (Na_3/4K_1/4)AlGeO₄ corresponds to a minor distortion of the nepheline structure. Chemical analysis by electron microprobe and structure determination of flux-grown single crystals indicate that the hexagonal form with the chemical formula (Na_0.78K_0.19)Al_0.97Ge_1.03O₄ may be stabilized by an alkali deficiency similar to that found in hexagonal natural nephelines. In contrast, all alkali sites are fully occupied in the monoclinic form of composition (Na_0.75K_0.25)AlGeO₄ and the lower symmetry eliminates the oxygen disorder present in the hexagonal form.

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