Synchrotron X-ray absorption spectroscopy and X-ray powder diffraction studies of the structure of johnbaumite [Ca_(10)(AsO_4)_6(OH,F)_2] and synthetic Pb-, Sr- and Ba-arsenate apatites and some comments on the crystal chemistry of the apatite structure type in general

摘要

The chemical composition of the natural arsenate-apatite mineral johnbaumite [nominally Ca_(10)(AsO_4)_6(OH)_2] and its alteration product hedyphane [Ca_4Pb_6(AsO_4)_6Cl_2] have been determined by electron microprobe analysis and the structures of johnbaumite and synthetic Sr-, Ba- and Pb-arsenate apatites have been studied by As K-edge X-ray absorption spectroscopy and synchrotron X-ray powder diffraction. All samples belong to the holosymmetric apatite space group P6_3/m with As~(5+) substituted for P~(5+) in the tetrahedral structural site. Johnbaumite contains small amounts of F and Pb (approx 0.9 and approx 4.4 wt. percent respectively) and hedyphane has the ideal composition (formula given above); the compositions of these coexisting phases define the two limbs of a solvus occurring between Ca- and Pb-arsenate apatite end members. The unit-cell parameters and cation-oxygen bond lengths for the arsenate apatites studied are discussed alongside published data for end-member Ca-, Sr-, Ba- and Pb-phosphate apatite analogues with (OH), F, Cl or Br as the anions at the centres of the channels in the apatite structure. This discussion rationalizes the relationships between the two structural sites A(1) and A(2) occupied by divalent cations in terms of the size of the A-O polyhedra and the distortion of the A4(1)-O polyhedron as measured by the metaprism twist angle [O(1)-A4(1)-O(2) projected onto (001)].