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New structural data reveal benleonardite to be a member of the pearceite-polybasite group

Published online by Cambridge University Press:  02 January 2018

Luca Bindi ,
Christopher J. Stanley  and
Paul G. Spry
Luca Bindi*
Affiliation:
Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy
Christopher J. Stanley
Affiliation:
Natural History Museum, Cromwell Road, London SW7 5BD, UK
Paul G. Spry
Affiliation:
Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA
*
*E-mail: luca.bindi@unifi.it
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Abstract

The determination of the crystal structure of benleonardite (P3m1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag15.00Cu1.00Sb1.58As0.42S7.03Te3.97) obtained using data from a gem-quality, untwinned crystal recovered from the type material, revealed that benleonardite exhibits the structure observed for minerals of the pearceite-polybasite group. The structure consists of the stacking of [Ag6(Sb,As)2S6Te]2– A and [Ag9Cu(S,Te)2Te2]2+B layer modules in which (Sb, As) forms isolated SbS3 pyramids typically occurring in sulfosalts; Cu links two (S,Te) atoms with linear coordination, and Ag occupies sites with coordination geometries ranging from quasi-linear to almost triangular. The silver ions are found in the B layer module along two-dimensional diffusion paths and their electron densities are evidenced by means of a combination of a Gram-Charlier development of the atom displacement factors and a split model. In the structure, two S positions are completely replaced by Te (i.e. Te3 and Te4) and one is half occupied [S1: S0.514(9)Te0.486], whereas S2 is completely filled by sulfur. This distribution reflects the crystal-chemical environments of the different cations. On the basis of information gained from this characterization, the crystal-chemical formula of benleonardite was revised according to the structural results, yielding Ag15Cu(Sb,As)2S7Te4 (Z = 1) instead of Ag8(Sb,As)Te2S3(Z = 2) as previously reported. Thus, the mineral must be considered a member of the pearceite-polybasite group. A recalculation of the chemical data listed in the scientific literature for benleonardite according to the structural results obtained here leads to excellent agreement.

Keywords

benleonarditecrystal structurepearceitepolybasiteAg-sulfosaltBambolla
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 5 , October 2015 , pp. 1213 - 1221
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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