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The crystal structure of faustite and its copper analogue turquoise

Published online by Cambridge University Press:  05 July 2018

U. Kolitsch  and
G. Giester
U. Kolitsch*
Affiliation:
Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A-1090 Wien, Austria
G. Giester
Affiliation:
Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A-1090 Wien, Austria
*
*E-mail: uwe.kolitsch@univie.ac.at
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Abstract

The crystal structure of faustite, ZnAl6(PO4)4(OH)8.4H2O, was determined using single-crystal data (Mo-Kα X-radiation, CCD area detector, 1624 unique reflections, R1 = 4.91%, wR2 = 9.23%), and compared with results of a reinvestigation of the structure of its copper analogue turquoise, CuAl6(PO4)4(OH)8.4H2O (2737 unique reflections, R1 = 2.81%, wR2 = 6.90%). Both are isostructural and crystallize in space group P, with a = 7.419(2) [turquoise: 7.410(1)], b = 7.629(3) [7.633(1)], c = 9.905(3) [9.904(1)] Å, α = 69.17(2) [68.42(1)], β = 69.88(2) [69.65(1)], γ = 64.98(2) [65.05(1)]°, V = 462.2(3) [460.27(10)] Å3, and Z = 1. The structure consists of distorted MO6 polyhedra (M = Zn, Cu), AlO6 octahedra and PO4 tetrahedra. By edge- and corner-sharing of these polyhedra a fairly dense three-dimensional framework is formed which is further strengthened by a system of hydrogen bonds. The metal atoms in the unique MO6 (M = Zn or Cu) polyhedron show a distorted [2+2+2]-coordination, the distortion being more pronounced in turquoise. About 10% of the M site is vacant in both minerals. In turquoise, a previously undetected structural site with a very low occupancy of (possibly) Cu is present at the position (Ý,0,Ý).

Keywords

faustiteturquoisecrystal structurecrystal chemistryphosphates
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 5 , October 2000 , pp. 905 - 913
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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