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Redefinition of arhbarite, Cu₂Mg(AsO₄)(OH)₃

¹ Henriette-Lott-Weg 8, D-50354 Hürth, Germany
² Institut für Mineralogie, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany
³ Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria

^* E-mail: We.Krause{at}t-online.de

Arhbarite was described in 1982 as a new copper arsenate, Cu₂(OH) AsO₄·6H₂O, from the Arhbar mine, Morocco. Due to the very small grain size no crystallographic data other than the X-ray powder diffraction data could be given. Reinvestigation of the type material and of new finds from the El Guanaco mine, Chile, is based on Rietveld refinement of X-ray powder data, electron microprobe investigations, optical studies as well as infrared and Raman spectra. The main results are: (1) arhbarite has the ideal formula Cu₂ Mg(AsO₄) (OH)₃ with magnesium oxide as an additional constituent and (2) arhbarite is isotypic with gilmarite, Cu₃ (AsO₄) (OH)₃. Arhbarite is triclinic, space group P1 (no. 1), unit-cell parameters (type material; refined from powder data) a = 5.315(4), b = 5.978(6), c = 5.030(6) Å , = 113.58(6), ß = 97.14(7), = 89.30(8)°, V = 145.2(1) ų, Z = 1. Microprobe analyses of the type material gave MgO 10.20, CuO 47.46, NiO 0.20, CoO 0.24, As₂ O₅ 33.85, SiO₂ 0.10, H₂O 7.58 (calc.), total 99.63 wt.%. The empirical formula is Cu_1.98(Mg_0.88Cu_0.09Ni_0.01Co_0.01)_0.99(AsO₄)_1.02(OH)_2.92, based on 7 O atoms. The Chilean arhbarite shows a slightly higher Cu content than the type material; it has the formula Cu_1.99 (Mg_0.73 Cu_0.25 Zn_0.01) _0.99 [(AsO₄)_1.01 (SiO₄)_0.01]_1.02(OH)_2.92. The formal redefinition of arhbarite has been approved by the CNMMN of the IMA.

KEYWORDS: arhbarite, redefinition, Rietveld refinement, microprobe data, infrared spectra, Raman spectra, crystal chemistry, gilmarite

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