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

Published online by Cambridge University Press:  05 July 2018

W. Krause ,
H.-J. Bernhardt ,
H. Effenberger ,
U. Kolitsch  and
CH. Lengauer
W. Krause*
Affiliation:
Henriette-Lott-Weg 8, D-50354 Hürth, Germany
H.-J. Bernhardt
Affiliation:
Institut für Mineralogie, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany
H. Effenberger
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria
U. Kolitsch
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria
CH. Lengauer
Affiliation:
Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria
*
* E-mail: We.Krause@t-online.de
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Abstract

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 Cu2Mg(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, α = 113.58(6), β = 97.14(7), γ = 89.30(8)°, V= 145.2(1)Å3, Z= 1. Microprobe analyses of the type material gave MgO 10.20, CuO 47.46, NiO 0.20, CoO 0.24, As2O5 33.85, SiO2 0.10, H2O 7.58 (calc.), total 99.63 wt.%. The empirical formula is Cu1.98(Mg0.88Cu0.09Ni0.01Co0.01)S0.99(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 Cu1.99(Mg0.73Cu0.25Zn0.01)∑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

arhbariteredefinitionRietveld refinementmicroprobe datainfrared spectraRaman spectracrystal chemistrygilmarite.
Type
Research Article
Information
Mineralogical Magazine , Volume 67 , Issue 5 , October 2003 , pp. 1099 - 1107
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2003

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