GAULTITE, A NEW ZEOLITE-LIKE MINERAL SPECIES FROM MONT-SAINT-HILAIRE,QUEBEC, AND ITS CRYSTAL-STRUCTURE

The new mineral species gaultite occurs as euhedral, multifaceted crys tals to 0.5 mm in a sodalite inclusion at the contact of hornfels and nepheline-sodalite syenite, Poudrette Quarry, Mont Saint-Hilaire, Rouv ille County, Quebec. It is colorless to pale mauve, has a white streak , vitreous luster and a bright apple-green fluorescence under short-wa ve ultraviolet radiation. The measured density is 2.52(4) g cm(-3); th e hardness is 6. It is biaxial (+), alpha 1.520(1), beta 1.521(1), gam ma 1.524(1), 2V 61.3(4)degrees (590 nm), is nonpleochroic, with X para llel to a, Y parallel to c and Z parallel to b. The average chemical c omposition (wt.%) is: Na2O 16.6, ZnO 19.2, SiO2 52.6, H2O(calc.) 11.3, total 99.7. The empirical formula is Na4.28Zn1.88Si6.99O18.5H(2)O whi ch, considered with structural data, gives the ideal formula Na4Zn2Si7 O18.5H(2)O (Z = 8). The strongest six lines of the X-ray powder patter n [d in Angstrom, 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) Angstrom, V 4196(2) Angstrom(3), space group F2dd. Th e crystal structure of gaultite was solved by Patterson methods; refin ement of the structure using 1210 observed reflections (MoK alpha) con verged to R = 2.5, wR = 2.8%. Gaultite shows no chemical disorder. The re are five tetrahedral sites; four of these are occupied by Si, and o ne by Zn. Polymerization of the tetrahedra results in a framework base d on a complex three-dimensional stacking of 4.8(2) Schlafli nets. Na occurs in two sites that represent voids in the framework of tetrahedr a. Nal is [6]-coordinated and positionally ordered; Na2 is positionall y disordered, split into [5]- and [6]-coordinated sites. Bond-valence analysis unambiguously shows three H2O groups, two of which are ordere d, 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 w hich may be the synthetic analogue of gaultite.