The sulfite anion in ettringite-group minerals: a new mineral species hielscherite, Ca3Si(OH)6(SO4)(SO3)•11H2O, and the thaumasite-hielscherite solid-solution series

Hielscherite, ideally Ca3Si(OH)(6)(SO4)(SO3)center dot 11H(2)O, (IMA 2011-037) is the first ettringite-group mineral with essential sulfite. We have identified a continuous natural solid-solution series from endmember thaumasite, Ca3Si(OH)(6)(SO4)(CO3)center dot 12H(2)O, to a composition with at least 77 mol.% endmember hielscherite. In this series, the SO3:CO3 ratio is variable, whereas the SO4 content remains constant. Compositions with more than 50 mol.% endmember hielscherite have only been found at Graulay quarry near Hillesheim in the western Eifel Mountains, Rhineland-Palatinate, where they occur with phillipsite-K, chabazite-Ca and gypsum in cavities in alkaline basalt. Sulfite-rich thaumasite has been found in hydrothermal assemblages in young alkaline basalts in two volcanic regions of Germany: it is widespread at Graulay quarry and occurs at Rother Kopf, Schellkopf and Bellerberg quarries in Eifel district; it has also been found at Zeilberg quarry, Franconia, Bavaria. Hielscherite forms matted fibrous aggregates up to 1 cm across and groups of acicular to prismatic hexagonal crystals up to 0.3 x 0.3 x 1.5 mm. Individual crystals are colourless and transparent with a vitreous lustre and crystal aggregates are white with a silky lustre. The Mohs hardness is 2-2 1/2. Measured and calculated densities are D-meas = 1.82(3) and D-calc = 1.79 g cm(-3). Hielscherite is optically uniaxial (-), omega = 1.494(2), epsilon = 1.476(2). The mean chemical composition of holotype material (determined by electron microprobe for Ca, Al, Si, and S and gas chromatography for C, H and N, with the S4+:S6+ ratio from the crystal-structure data) is CaO 27.15, Al2O3 2.33, SiO2 7.04, CO2 2.71, SO2 6.40, SO3 12.91, N2O5 0.42, H2O 39.22, total 98.18 wt.%. The empirical formula on the basis of 3 Ca atoms per formula unit is Ca-3(Si0.73Al0.28)(Sigma 1.01)(OH)(5.71)(SO4)(1.00)(SO3)(0.62)(CO3)(0.38)(NO3)(0.05)center dot 10.63H(2)O. The presence of sulfite was confirmed by crystal-structure analysis and infrared and X-ray absorption near edge structure spectra. The crystal structure of sulfite-rich thaumasite from Zeilberg quarry was solved by direct methods based on single-crystal X-ray diffraction data (R-1 = 0.064). The structure of hielscherite was refined using the Rietveld method (R-wp = 0.0317). Hielscherite is hexagonal, P6(3), a = 11.1178(2), c = 10.5381(2) angstrom, V = 1128.06(4) angstrom(3) and Z = 2. The strongest reflections in the X-ray powder pattern [(d,angstrom(I)(hkI)] are: 9.62(100)(010,100); 5.551(50)(110); 4.616(37)(012,102); 3.823(64)(112); 3.436(25)(211), 2.742(38)(032,302), 2.528(37)(123,213), 2.180(35)(042,402;223). In both hielscherite and sulfite-rich thaufnasite, pyramidal sulfite groups occupy the same site as trigonal carbonate groups, with analogous O sites, whereas tetrahedral sulfate groups occupy separate positions. Hielscherite is named in honour of the German mineral collector Klaus Hielscher (b. 1957).