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

<jats:title>Abstract</jats:title><jats:p>Hielscherite, ideally Ca<jats:sub>3</jats:sub>Si(OH)<jats:sub>6</jats:sub>(SO<jats:sub>4</jats:sub>)(SO<jats:sub>3</jats:sub>)·11H<jats:sub>2</jats:sub>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, Ca<jats:sub>3</jats:sub>Si(OH)<jats:sub>6</jats:sub>(SO<jats:sub>4</jats:sub>)(CO<jats:sub>3</jats:sub>)·12H<jats:sub>2</jats:sub>O, to a composition with at least 77 mol.% endmember hielscherite. In this series, the SO3:CO3 ratio is variable, whereas the SO<jats:sub>4</jats:sub> 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<jats:sub>meas</jats:sub> = 1.82(3) and <jats:italic>D</jats:italic><jats:sub>calc</jats:sub> = 1.79 g cm<jats:sup>–3</jats:sup>. 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 S<jats:sup>4+</jats:sup>:S<jats:sup>6+</jats:sup> ratio from the crystal-structure data) is CaO 27.15, Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 2.33, SiO<jats:sub>2</jats:sub> 7.04, CO<jats:sub>2</jats:sub> 2.71, SO<jats:sub>2</jats:sub> 6.40, SO<jats:sub>3</jats:sub> 12.91, N<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub> 0.42, H<jats:sub>2</jats:sub>O 39.22, total 98.18 wt.%. The empirical formula on the basis of 3 Ca atoms per formula unit is Ca<jats:sub>3</jats:sub>(Si<jats:sub>0.73</jats:sub>Al<jats:sub>0.28</jats:sub>)<jats:sub>Σ1.01</jats:sub>(OH)<jats:sub>5.71</jats:sub>(SO<jats:sub>4</jats:sub>)<jats:sub>1.00</jats:sub>(SO<jats:sub>3</jats:sub>)<jats:sub>0.62</jats:sub>(CO<jats:sub>3</jats:sub>)<jats:sub>0.38</jats:sub>(NO<jats:sub>3</jats:sub>)<jats:sub>0.05</jats:sub>·10.63H<jats:sub>2</jats:sub>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<jats:sub>1</jats:sub> = 0.064). The structure of hielscherite was refined using the Rietveld method (R<jats:sub>wp</jats:sub> = 0.0317). Hielscherite is hexagonal, P6<jats:sub>3</jats:sub>, a = 11.1178(2), c = 10.5381(2) Å, V = 1128.06(4) Å3 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).</jats:p>