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Structural role of copper in the minerals of the pearceite-polybasite group: the case of the new minerals cupropearceite and cupropolybasite

¹ Museo di Storia Naturale, sezione di Mineralogia, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
² Laboratoire de Chimie des Solides, I.M.N., UMR 6502 CNRS - Université de Nantes, 2 rue de la Houssinière, B.P. 32229, F-44322 Nantes Cedex 3, France
³ Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA
⁴ Department of Natural History, Royal Ontario Museum, 100 Queens Park, Toronto, Ontario M5S 2C6, Canada
⁵ Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy

^* E-mail: luca.bindi{at}unifi.it

The pearceite-polybasite group of minerals, general formula [M₆T₂S₇][Ag₉CuS₄] with M = Ag, Cu; and T = As, Sb, show a crystal structure which can be described as the succession, along the c axis, of two pseudo-layer modules: a [M₆T₂S₇]^2– A module layer and a [Ag₉CuS₄]²⁺ B module layer. Copper is present in one structural position of the B module layer and replaces Ag in the only fully occupied M position of the A module layer. When the Cu content is >4.00 a.p.f.u., the structural position of the A module layer becomes Cu-dominant and, consequently, the mineral deserves its own name. In this paper we report the crystal-chemical characterization of two Cu-rich members exhibiting the 111 unit-cell type (corresponding to the Tac polytype). One sample (space group Pm1, a 7.3218(8), c 11.8877(13) Å, V 551.90(10) ų, Z = 1) having As >Sb and with the structural position of the A module layer dominated by Cu, has been named cupropearceite and the other sample (space group Pm1, a 7.3277(3), c 11.7752(6) Å, V 547.56(8) ų, Z = 1) having Sb >As has been named cupropolybasite.

Both the new minerals and mineral names have been approved by the IMA-CNMNC.

KEYWORDS: cupropearceite, cupropolybasite, new mineral, crystal structure, chemical composition