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American Mineralogist; May-June; v. 94; no. 5-6; p. 676-683; DOI: 10.2138/am.2009.3083
© 2009 Mineralogical Society of America
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The crystal chemical role of Zn in alunite-type minerals: Structure refinements for kintoreite and zincian kintoreite

Ian E. Grey1,*, W. Gus Mumme1, Stuart J. Mills2, William D. Birch3 and Nicholas C. Wilson1

1 CSIRO Minerals, Box 312, Clayton South, Victoria 3169, Australia
2 University of British Columbia, Department of Earth and Ocean Sciences, Vancouver, British Columbia V6T 1Z4, Canada
3 Geosciences, Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia

Correspondence: * E-mail: ian.grey{at}csiro.au

Kintoreite, PbFe3H0.94[(PO4)0.97(SO4)0.03]2(OH)6, and zincian kintoreite, PbZn0.3Fe3H0.24[(PO4)0.54 (SO4)0.08(AsO4)0.38]2(OH)6, have rhombohedral symmetry, space group RFormulam, with hexagonal cell parameters a = 7.2963(5) Å, c = 16.8491(5) Å, and a = 7.3789(3) Å, c = 16.8552(7) Å, respectively. The structures have been refined using single-crystal X-ray data to R1 = 0.030 for 374 observed reflections and R1 = 0.035 for 399 observed reflections, respectively. The structures of both minerals comprise rhombohedral stacking of (001) composite layers of corner-shared octahedra and tetrahedra with Pb atoms occupying icosahedral sites between the layers, as in the alunite-type structure. The corner-connected octahedra form three-membered and six-membered rings as in hexagonal tungsten bronzes. The structure of zincian kintoreite differs from other alunite-type structures in having partial occupation, by Zn, of new sites within the six-membered rings in the octahedral layers. The Zn is displaced to an off-center position in the hexagonal ring, where it assumes fivefold trigonal-bipyramidal coordination, to three of the hydroxyl anions forming the ring, and to the apical O anions of the XO4 tetrahedra on opposite sides of the ring. The different structural modes of Zn incorporation into SO4-dominant and (P,As)O4-dominant members of A2+B33+(XO4)2(OH)6 alunite-type minerals are discussed in terms of the different charge-compensation mechanisms involved.

Key Words: Zincian kintoreite • crystal structure • alunite-type structure • single-crystal study






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