Mallestigite, Pb3Sb(SO4)(AsO4)(OH)6·3H2O, from the type locality – new data, crystal structure, and structural relationships

A report of the supergene mineral mallestigite, Pb₃Sb⁵⁺(SO₄)(AsO₄)(OH)₆·3H₂O, from the type locality Neufinkenstein-Grabanz district, Mallestiger Mittagskogel, Carinthia, Austria, is given in order to close gaps that have been left open since its first description. At the type locality, the dump of an abandoned small copper-lead-zinc mine, the mineral occurs frequently in up to ⁓2 mm long clear colourless idiomorphic hexagonal prisms {100} terminated by the bipyramid {101}. Details of the occurrence, paragenesis, physical properties, a FTIR spectrum and X-ray powder diffraction data are reported. Chemical analysis in a scanning electron microscope in conjunction with a crystal structure analysis gave the empirical formula Pb_3.02Sb_1.06(SO₄)_0.96(AsO₄)_0.97(OH)₆·3H₂O. An ab-initio crystal structure determination with CCD diffractometer data, MoKα radiation, and a nearly untwinned crystal proved that mallestigite is hexagonal, space group P6₃, a = 8.9326(4), c = 11.1044(5) Å, V = 767.33(8) ų, Z = 2, and gave a final R1 = 0.0184 for 1632 independent reflections with I > 2σ(I). Mallestigite belongs to the fleischerite group. The structure is built up from [Sb⁵⁺(OH)₆]^1– octahedra which are linked by triplets of edge-sharing Pb²⁺ into chains [Pb₃Sb(OH)₆]⁵⁺ extending along a 6₃ axis. Each three of such chains are bridged by triplets of H₂O and by rows of SO₄^2– and AsO₄^3– tetrahedra alternating along a 3-fold axis and showing very different links to the Pb atom. Pb²⁺ is in a strongly distorted one-sided [9]-coordination by O atoms that makes the structure clearly hemimorphic. Structural relationships to the fleischerite group minerals schaurteite, genplesite, despujolsite and fleischerite as well as to synthetic Ba₃Sn⁴⁺(SeO₄)₂(OH)₆·3H₂O are outlined and changes in the space group symmetry of despujolsite (P\(\overline6\)2c → P6₃/mmc) and fleischerite (P\(\overline6\)2c → P6₃) are proposed.