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Gaultite, a new zeolite-like mineral species from Mont Saint-Hilaire, Quebec, and its crystal structure

1994; Mineralogical Association of Canada; Volume: 32; Issue: 4 Linguagem: Inglês

1499-1276

T. S. Ercit, Jerry Van Velthuizen,

Glass properties and applications

The new mineral species gaultite occurs as euhedral, multifaceted crystals to 0.5 mm in a sodalite inclusion at the contact of hornfels and nepheline-sodalite syenite, Poudrette Quarry, Mont Saint-Hilaire, Rouville County, Quebec. It is colorless to pale mauve, has a white streak, vitreous luster and a bright apple-green fluorescence under short-wave ultraviolet radiation. The measured density is 2.52(4) g cm- 3 ; the hardness is 6. It is biaxial (+), a 1.520(1), ~ 1.521(1), Y 1.524(1), 2V 61.3(4)° (590 om), is nonpleochroic, with X II a, YII c and Z II b. The average chemical composition (wt.%) is: NazO 16.6, ZnO 19.2, Si02 52.6, H20(calc.) 11.3, total 99.7. The empirical formula is Na4.2SZn1.ssSi6.9901S·5H20 which, considered with structural data, gives the ideal formula Na4Zn2Si701g5H20 (Z = 8). The strongest six lines of the X-ray powder pattern [d in A, I in %] are 6.35(100), 4.96(30), 3.24(60), 3.17(40), 3.14(40), 2.82(30). The unit-cell parameters are a 10.211(3), b 39.88(2), c 10.304(4) A, V 4196(2) A3, space group F2dd. The crystal structure of gaultite was solved by Patterson methods; refinement of the structure using 1210 observed reflections (MoKa) converged to R = 2.5, wR = 2.8%. Gaultite shows no chemical disorder. There are five tetrahedral sites; four of these are occupied by Si, and one by Zn. Polymerization of the tetrahedra results in a framework based on a complex three-dimensional stacking of 4.8 2 Schliifli nets. Na occurs in two sites that represent voids in the framework of tetrahedra. Na1 is [6]-coordinated and positionally ordered; Na2 is positionally disordered, split into [5]- and [6]-coordinated sites. Bond-valence analysis unambiguously shows three H20 groups, two of which are ordered, with ordered protons, one of which is disordered, in agreement with the results of IR spectroscopy. The structure of gaultite is similar to those of lovdarite and the synthetic zeolite VPI-7, the latter of which may be the synthetic analogue of gaultite.