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Synchrotron X-ray absorption spectroscopy and X-ray powder diffraction studies of the structure of johnbaumite [Ca10(AsO4)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

Published online by Cambridge University Press:  05 July 2018

C. M. B. Henderson ,
A. M. T. Bell ,
J. M. Charnock ,
K. S. Knight ,
R. F. Wendlandt ,
D. A. Plant  and
W. J. Harrison
C. M. B. Henderson*
Affiliation:
School ofEarth, Atmospheric and Environmental Sciences (SEAES), University ofManchester, Manchester M13 9PL, UK Photon Science Department, STFC Daresbury Laboratory, Warrington WA4 4AD, UK
A. M. T. Bell
Affiliation:
Photon Science Department, STFC Daresbury Laboratory, Warrington WA4 4AD, UK
J. M. Charnock
Affiliation:
School ofEarth, Atmospheric and Environmental Sciences (SEAES), University ofManchester, Manchester M13 9PL, UK
K. S. Knight
Affiliation:
ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK
R. F. Wendlandt
Affiliation:
Colorado School ofMines, Golden, Colorado 80401, USA
D. A. Plant
Affiliation:
School ofEarth, Atmospheric and Environmental Sciences (SEAES), University ofManchester, Manchester M13 9PL, UK
W. J. Harrison
Affiliation:
Colorado School ofMines, Golden, Colorado 80401, USA
*
* E-mail: michael.henderson@manchester.ac.uk
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Abstract

The chemical composition oft he natural arsenate-apatite mineral johnbaumite [nominally Ca10(AsO4)6(OH)2] and its alteration product hedyphane [Ca4Pb6(AsO4)6Cl2] have been determined by electron microprobe analysis and the structures ofjohnbaumite and synthetic Sr-, Ba- and Pbarsenate 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 P63/m with As5+ substituted for P5+ in the tetrahedral structural site. Johnbaumite contains small amounts ofF and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions of these coexisting phases define the two limbs ofa 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 Pbphosphate 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 A(1)–O polyhedron as measured by the metaprism twist angle [O(1)–A(1)–O(2) projected onto (001)].

Keywords

Arsenate apatitesjohnbaumiteX-ray structure determinationcrystal chemistry.
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 3 , June 2009 , pp. 433 - 455
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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