Referenza:
▪ Kolitsch, U. & Topa, D. (2021): (Ag4.38Cu2.62)S7.00AsS5: A new silver-copper sulphosalt from the Bou Azzer Mining District, Morocco. Poster, MinPet 2021, Vienna, Austria, September 19-21; Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 167, 117.
Abstract:
We have characterised a new silver-copper sulphosalt species from Vein no. 53 (‘Filon 53’), Aït Ahmane (Bou Azzer mining district), Zagora Province, Drâa-Tafilalet Region, Morocco, by quantitative electron microprobe data, a single-crystal structure determination and reflected-light studies. The new mineral, of which only one specimen is presently known to exist, forms black, elongate pseudo-orthorhombic prisms (up to 0.5 mm in length) with a strong metallic lustre. The crystals show wedge-shaped crystal terminations (similar to löllingite), strong mosaic growth and are arranged in sprays in a void in a white carbonate (probably calcite). The new mineral is intimately associated with tiny dark red, six-sided platelets of earlier crystallised cupropearceite with the empirical formula Ag11.39Cu4.67As1.97S10.96 (microprobe data). The black prisms were briefly described, along with preliminary SEM-EDS data, as “UK26 - cupropearceite?” in a collectors’ journal (FAVREAU & BARRAL, 2019).
The solution of the structure (R = 7.6%) from single-crystal XRD data (MoKα; 293 K) unambiguously indicated space group P21/n (no. 14), with a = 7.175(1), b = 13.411(3), c = 10.048(2) Å, beta = 91.49(3)°, V = 966.5(3) Å3 (Z = 4), with the refined chemical formula (Ag4.38Cu2.62)S7.00AsS5, i.e. an Ag:Cu ratio of 1.67. The microprobe data, which were obtained from a spray at the edge of the void, show a lower Ag:Cu ratio (1.48; range: 1.41-1.54). They confirm the complete absence of Sb. Optically, the polished crystals are homogeneous, with no indication of twin lamellae or other anomalies. The asssociated cupropearceite has a distinctly higher Ag:Cu ratio (2.44).
The asymmetric unit contains seven metal sites, one As site and four S sites. The metal sites comprise three Ag sites, one mixed (Ag0.91(4)Cu0.09) site, one Cu site and two (Cu,Ag) sites with occupancies (Cu0.89(3)Ag0.11) and (Cu0.65(3)Ag0.35). The formula is charge-balanced and no As–As or distinct metal–metal bonds exist. Ag–S bond lengths range between 2.45 and 2.80 Å, while the Cu–S and (Cu,Ag)–S bond lengths show a range between 2.21 and 2.55 Å. The As–S bonds are between 2.23 and 2.26 Å long. The atomic arrangement can be described as a 3D framework composed of metal-Sn (n = 2-4) polyhedra and AsS3 pyramids.
Comparisons will be drawn with chemically related species.
We thank Jean-Pierre Barral for providing the specimen (via Philippe Roth) for analysis.
▪ Favreau, G. & Barral, J.-P. (2019): Le Cahier des Micromonteurs, 145, 3-129 (in French).
(Ag4.38Cu2.62)S7.00AsS5: A new silver-copper sulphosalt from the Bou Azzer Mining District, Morocco
- Marco E. Ciriotti
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(Ag4.38Cu2.62)S7.00AsS5: A new silver-copper sulphosalt from the Bou Azzer Mining District, Morocco
Marco E. Ciriotti
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