The crystal chemical role of Zn in alunite-type minerals: Structure refinements for kintoreite and zincian kintoreite
-
Ian E. Grey
,
W. Gus Mumme
Abstract
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 R3m, 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 cornerconnected 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.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
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- Arsenate substitution in hydroxylapatite: Structural characterization of the Ca5(PxAs1–xO4)3OH solid solution
- The crystal chemical role of Zn in alunite-type minerals: Structure refinements for kintoreite and zincian kintoreite
- Application of precession electron diffraction to the characterization of (021) twinning in pseudo-hexagonal coesite
- Structure, thermodynamic, and transport properties of molten Mg2SiO4: Molecular dynamics simulations and model EOS
- High-pressure behavior of åkermanite and gehlenite and phase stability of the normal structure in melilites
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- Crystal chemistry of hydrous forsterite and its vibrational properties up to 41 GPa
- The geothermobarometric potential of tourmaline, based on experimental and natural data
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- Letter. Davisite, CaScAlSiO6, a new pyroxene from the Allende meteorite
- Letter. The 4500 cm–1 infrared absorption band in hydrous aluminosilicate glasses is a combination band of the fundamental (Si,Al)-OH and O-H vibrations
