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Lead-antimony sulfosalts from Tuscany (Italy). XVI. Carducciite, (AgSb)Pb6(As,Sb)8S20, a new Sb-rich derivative of rathite from the Pollone mine, Valdicastello Carducci: occurrence and crystal structure

Published online by Cambridge University Press:  05 July 2018

Cristian Biagioni ,
Paolo Orlandi ,
Yves Moëlo  and
Luca Bindi
Cristian Biagioni*
Affiliation:
Dipartimento di Scienze della Terra, Universita` di Pisa, Via S. Maria 53, I-56126 Pisa, Italy
Paolo Orlandi
Affiliation:
Dipartimento di Scienze della Terra, Universita` di Pisa, Via S. Maria 53, I-56126 Pisa, Italy
Yves Moëlo
Affiliation:
Institut des Matériaux Jean Rouxel, UMR 6502, CNRS, Université de Nantes, 2, rue de la Houssiniére, 44 322 Nantes Cedex 3, France
Luca Bindi
Affiliation:
Dipartimento di Scienze della Terra, Universita` di Firenze, Via G. La Pira 4, I-50121 Florence, Italy
*
* E-mail: biagioni@dst.unipi.it
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Abstract

The new mineral species carducciite, (AgSb)Pb6(As,Sb)8S20, has been discovered in the baryte-pyrite- (Pb-Ag-Zn) deposit of the Pollone mine, near Valdicastello Carducci, Apuan Alps, Tuscany, Italy. It occurs as black metallic prismatic crystals, up to 0.5 mm long, associated with pyrite and sterryite. Its Vickers hardness (VHN10) is 61 kg/mm2 (range: 52–66), corresponding to a Mohs hardness of ~2½–3. In reflected light, carducciite is dark grey in colour, moderately bireflectant; internal reflections are very weak and deep red in colour. Reflectance percentages for the four COM wavelengths [Rmin, Rmax (%) (λ)] are: 35.8, 40.8 (471.1 nm), 33.7, 39.0 (548.3 nm), 32.7, 37.6 (586.6 nm) and 30.4, 35.1 (652.3 nm). Electron microprobe analysis gives (wt.% – mean of six analyses): Ag 3.55(12), Tl 0.13(3), Pb 41.90(42), Sb 17.79(19), As 12.41(14), S 22.10(17), total 97.9(6). On the basis of ΣMe = 16 a.p.f.u., the chemical formula is Ag0.96Tl0.02Pb5.91As4.84Sb4.27S20.14. The main diffraction lines, corresponding to multiple hkl indices, are (relative visual intensity): 3.689 (s), 3.416 (s), 3.125 (s), 2.989 (s), 2.894 (s), 2.753 (vs), 2.250 (s). The crystal-structure study gives a monoclinic unit cell, space group P21/c, with a 8.4909(3), b 8.0227(3), c 25.3957(9) Å, β 100.382(2)°, V 1701.63(11) Å3, Z = 2. The crystal structure has been solved and refined to a final R1 = 0.063 on the basis of 4137 observed reflections. It can be described within the framework of the sartorite homologous series, as formed by chemically twinned layers of the dufrénoysite type. The simplified idealized structural formula, based on 20 sulfur atoms, can ideally be written as (AgSb)Pb6(As,Sb)Σ=8S20. Carducciite is an (Ag,Sb)-rich homeotype of dufrénoysite, stabilized by the complete coupled substitution 2 Pb2+ = Ag+ + Sb3+ on a specific site of the crystal structure. Together with barikaite, it belongs to the rathite sub-group of P21/c homeotypes of dufrénoysite, of which the crystal chemistry is discussed. The distribution of Ag, coupled with As or Sb on specific sites, appears to be the main criterion for the distinction between the three species of this sub-group.

Keywords

carducciitenew mineralrathitesulfosaltleadsilverantimonyarseniccrystal structurePollone mineApuan AlpsTuscanyItaly
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 7 , December 2014 , pp. 1775 - 1793
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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