Crystal structures and isotypism of the iron(III) arsenate kamarizaite and the iron(III) phosphate tinticite

摘要

The crystal structures of the secondary ferric iron minerals kamarizaite, Fe-3(3+)(AsO4)(2)(OH)(3)center dot 3H(2)O, and tinticite, Fe-3(3+)(PO4)(2)(OH)(3)center dot 3H(2)O, for which highly contradictory data on crystal symmetry were reported, were studied by a combination of single-crystal X-ray diffraction and Rietveld refinement (supplemented by chemical analyses and thermogravimetry), using type material of both species and additional samples from several other localities, including the type localities. The previously unknown crystal structure of kamarizaite was determined from single-crystal intensity data (MoK alpha, 293 K, R(F) = 2.91 ; all H atoms detected) using a sample from the Le Mazet vein, Echassieres, Auvergne, France. The mineral is triclinic, space group P (1) over bar (no. 2), with a = 7.671(2), b = 8.040(2), c = 10.180(2) angstrom, alpha = 68.31(3), beta = 75.35(3), gamma = 63.52(3)degrees, V = 519.3(2) angstrom(3), Z = 2. Rietveld analyses of fine-grained kamarizaite collected underground at two different spots in Lavrion, Greece (Hilarion and Jean Baptiste areas) confirmed the structure model. Rietveld analyses of fine-grained tinticite from Tintic, Utah (USA), Bruguers (Spain) and Weckersdorf (Germany) demonstrate that kamarizaite and tinticite are triclinic and isotypic. A previously published structure model for tinticite, as well as the originally reported orthorhombic symmetry for kamarizaite, are shown to be incorrect. Refined unit-cell parameters of a cotype tinticite specimen from Tintic are: a = 7.647(1), b = 7.958(1), c = 9.987(1) angstrom, alpha = 67.90(1), beta = 76.10(1), gamma = 64.10(1)degrees, V = 504.4(2) angstrom(3). Bruguers and Weckersdorf tinticite have very similar parameters. The common atomic arrangement is characterised by three unique, octahedrally coordinated Fe sites (on which Fe may be partially replaced by minor Al), two unique tetrahedrally coordinated T (As or P) sites, eight O, three O-h, three O-w and nine H sites. The topology features zig-zag chains along 110 of dimers built of two edge-sharing FeO6 octahedra corner-linked by a third FeO6 octahedron. The chains are corner-linked by the TO4 tetrahedra thus establishing a mixed octahedral-tetrahedral framework with a T:Fe ratio of 0.67, a pronounced layered arrangement parallel to (001) and narrow channels along 010. Medium-strong to weak hydrogen-bonds provide additional strengthening of the structure. The topology is closely related to that of the recently described, triclinic aluminium phosphate afmite.