• journal_title：Mineralogical Magazine
• Contributor：I. V. Pekov ; N. V. Chukanov ; S. N. Britvin ; Y. K. Kabalov ; J. Göttlicher ; V. O. Yapaskurt ; A. E. Zadov ; S. V. Krivovichev ; W. Schüller ; B. Ternes
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2012-10-01
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2012.076.5.06
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：76
• issue：5
• firstpage：1133
• section：Articles

Hielscherite, ideally Ca₃Si(OH)₆(SO₄)(SO₃)·11H₂O, (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)·12H₂O, 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 × 0.3 × 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 2–2½. Measured and calculated densities are D_means = 1.82(3) and D_calc = 1.79 g cm⁻³. Hielscherite is optically uniaxial (−), ω = 1.494(2), ε = 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 Ca₃(Si_0.73Al_0.28)_Σ1.01(OH)_5.71(SO₄)_1.00(SO₃)_0.62(CO₃)_0.38(NO₃)_0.05·10.63H₂O. 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 (R₁ = 0.064). The structure of hielscherite was refined using the Rietveld method (R_wp = 0.0317). Hielscherite is hexagonal, P6₃, a = 11.1178(2), c = 10.5381(2) Å, V = 1128.06(4) ų and Z = 2. The strongest reflections in the X-ray powder pattern [(d,Å(I)(hkl)] 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 thaumasite, 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).