Monoclinic and hexagonal nepheline structures of (Na3/4K1/4)AlGeO4

Hexagonal (Na3/4K1/4)AlGeO4 crystallizes in the space group P6(3) with unit-cell parameters a = 10.164 (2), c = 8.540 (2) Angstrom and Z = 8 [wR(F-2) = 0.066 for all 3060 independent reflections]. Monoclinic (Na3/4K1/4)AlGeO4 crystallizes in the space group P2(1) with unit-cell parameters a = 10.0477 (4), b = 8.5764 (4), c = 10.2118 (4) Angstrom, beta = 119.035(1)degrees and Z = 8 [wR(F-2) = 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 (Na3/4K1/4)AlGeO4 is isostructural with the silicate mineral nepheline (Na3/4K1/4)AlSiO4, while monoclinic (Na3/4K1/4)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 (Na0.78K0.19)Al0.97Ge1.03O4 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 (Na0.75K0.25)AlGeO4 and the lower symmetry eliminates the oxygen disorder present in the hexagonal form.