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The crystal structure of faustite and its copper analogue turquoise

Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A-1090 Wien, Austria

^* E-mail: uwe.kolitsch{at}univie.ac.at

The crystal structure of faustite, ZnAl₆(PO₄)₄(OH)₈·4H₂O, was determined using single-crystal data (Mo-K 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, = 69.17(2) [68.42(1)], ß = 69.88(2) [69.65(1)], = 64.98(2) [65.05(1)]°, V = 462.2(3) [460.27(10)] ų, 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 (,0,).

KEYWORDS: faustite, turquoise, crystal structure, crystal chemistry, phosphates

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