Home > Mineral Abstracts > CanMin Vol. 32, pp. 855-863 (1994)




Vol. 32, pp. 855-863 (1994)



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


T. Scott Ercit and Jerry van Velthuizen
Research Division, Canadian Museum of Nature,
Ottawa, Ontario K1P 6P4


ABSTRACT

     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, Québec. 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), b 1.521(1), g 1.524(1), 2V 61.3(4)° (590 nm), is nonpleochroic, with X = a, Y = c and Z = b. The average chemical composition (wt.%) is:


Na2O - 16.6
ZnO - 19.2
SiO2 - 52.6
H2O (calc.) - 11.3


     Total 99.7. The empirical formula is Na4.28Zn1.88Si6.99O18 · 5H2O (Z = 8) which, considered with structural data, gives the ideal formula Na4Zn2Si7O18 · 5H2O (Z = 8). The strongest six lines of the X-ray powder pattern [d in Å, l 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 10.21(3), b 39.88(2), c10.304(4) Å, V4196(2) Å3, 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 4.82 Schläfli 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 H2O groups, two of thich 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 VPI-7, the latter of which may be the synthetic analogue of gaultite.

Keywords:  gaultite, new mineral species, crystal structure, zeolite-like tectosilicates, Mont Saint-Hilaire, Québec.


© 1994  The Canadian Mineralogist