HILLITE, A NEW MEMBER OF THE FAIRFIELDITE GROUP: ITS DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Hillite ('(sic)' in Russian), ideally Ca-2(Zn,Mg)[PO4](2).2H(2)O, is a new zinc-dominant analogue of collinsite, isostructural with fairfieldite. It occurs in unmetamorphosed sediments. of the Lower Cambrian Parachilna Formation, Reaphook Hill, South Australia, Australia, in association with zincian collinsite and scholzite. Hillite is triclinic, a 5.736(1), b 6.767(2), c 5.462(1) Angstrom, alpha 97.41(2), beta 108.59(2), gamma 107.19(2)degrees V 186.05(8) Angstrom(3), space group P (1) over bar, Z = 1. The average result of fifteen electron-microprobe analyses of relatively homogeneous crystals of the latest generation is: Na2O 0.11, CaO 30.36, MgO 4.34, ZnO 14.79, FeO 0.04, P2O5 40.85. H2Ocalc 10.23 (calculated to give 2 H2O), total 100.72 wt.%. The single crystals of hillite do not exceed 50 mum in length: larger aggregates are rather inhomogeneous in terms of Zn:Mg ratio. Chemical analyses show that hillite contains as much as 64 mol.% of the Zn end-member, and there is perfect solid-solution toward collinsite. The mineral is colorless or grey with greenish or bluish tint, and shows weak greenish yellow fluorescence in short-wave ultraviolet light. It has a perfect cleavage along {010} and {001}: the Mohs hardness is 3(1)/(2). Hillite is biaxial positive with 2V(z) (calc.) = 83.4degrees, alpha 1.635(3), beta 1.650(5), gamma 1.669(3): D-meas = 3.16(2) g/cm(3). The six strongest X-ray powder-diffraction lines [d(meas) in Angstrom(I)(hkl)]: 6.24(34)(010), 3.230(22)(1 (1) over bar0), 3.130(37)(020) 3.038(40)(101), 2.690(100)(121) 2.230(14)(1 (3) over bar0). The crystal structure of hillite has been determined [automated single-crystal diffractometer, MoKalpha, graphite monochromator, image-plate area-detector system, T = 293 K 2928 reflections, wR(2) = 0.0998 for all 1078 unique reflections, R = 0.0378 for 993 observed reflections with I greater than or equal to 2sigma(I)]. The refinement of site occupancies showed that Zn and Mg are in the octahedral position at the center of symmetry in the proportion 0.623(5) : 0.377(5). in agreement with an empirical formula based on the electron-microprobe data. The positions of two independent H atoms were obtained from difference-Fourier syntheses and were refined under isotropic approximation. The structure of hillite consists of isolated MO4(H2O)(2) octahedra, which are connected by orthophosphate tetrahedra to form chains parallel to the c axis. In the a and b directions, these chains are held together by Ca atoms and hydrogen bonds.