Refinement of the crystal structure of zoned philipsbornite–hidalgoite from the Tsumeb mine, Namibia, and hydrogen bonding in the D2+G3+3(T5+O4)(TO3OH)(OH)6 alunite structures

Abstract

AbstractThe crystal structure of zoned philipsbornite – hidalgoite, hexagonal (rhombohedral), Rm , Z = 3: a = 7.1142(4), c = 17.0973(9) A ˚ , V = 749.4(1) Å3, from the Tsumeb mine, Namibia, has been refined to R1 = 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 Pb0.982+(Al2.92Fe0.132+)(AsO4)[(As0.72S0.27)O3.14(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 Pb0.992+(Al2.85Fe0.232+)(AsO4)[(As0.25S0.70)O3.30(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)O4] 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 D2+G33+(T5+O4)(TO3OH)(OH)6 minerals is proposed.