Refinement of the crystal structure of zoned philipsbornite-hidalgoite from the Tsumeb mine, Namibia, and hydrogen bonding in the D2+G33+(T5+O4)(TO3OH)(OH)6 alunite structures

The crystal structure of zoned philipsbornite-hidalgoite, hexagonal (rhombohedral), R (3) over barm, Z = 3: a = 7.1142(4), c = 17.0973(9) angstrom, V = 749.4(1) angstrom(3), from the Tsumeb mine, Namibia, has been refined to R-1 = 1.68% for 301 unique reflections collected on a Bruker D8 three-circle diffractometer equipped with a rotating-anode generator, multilayer optics and an APEX-II CCD detector. Chemical analysis by electron microprobe showed zoned crystals with a rim enriched in S and Fe relative to the core. The core composition is SO3 3.31, As2O5 30.57, Al2O3 23.05, FeO 1.44, PbO 33.94, H2Ocalc 9.58, total 101.79 wt.%, corresponding to Pb-0.98(2+)(Al2.92Fe0.132+)(AsO4)[(As0.72S0.27)O-3.14(OH)(0.85)](OH)(6); and the rim composition is SO3 8.88, As2O5 22.63, Al2O3 22.90, FeO 2.57, PbO 34.91, H2Ocalc 9.27, total 101.16 wt.%, corresponding to Pb-0.99(2+)(Al2.85Fe0.232+)(AsO4)[(As0.25S0.70)O-3.30(OH)(0.50)](OH)(6). Philipsbornite-hidalgoite has the alunite-type structure, sheets of corner-sharing octahedra, decorated on top and bottom by [(As,S)O-4] and (AsO3OH) tetrahedra, that are linked into a three-dimensional structure by [12]-coordinated Pb2+ cations and hydrogen bonds. A new hydrogen-bonding scheme for the D(2+)G(3)(3+)(T5+O4)(TO3OH)(OH)(6) minerals is proposed.