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

The new mineral species carducciite, (AgSb)Pb-6(As,Sb)(8)S-20, 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/mm(2) (range: 52 - 66), corresponding to a Mohs hardness of similar to 21/2 - 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 [R-min, R-max (%) (lambda)] 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 Sigma Me = 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 P2(1)/c, with a 8.4909(3), b 8.0227(3), c 25.3957(9) angstrom, beta 100.382(2)degrees, V 1701.63(11) angstrom, Z = 2. The crystal structure has been solved and refined to a final R-1 = 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 dufrenoysite type. The simplified idealized structural formula, based on 20 sulfur atoms, can ideally be written as (AgSb)Pb-6(As,Sb)(Sigma-8)S-20. Carducciite is an (Ag,Sb)-rich homeotype of dufrenoysite, 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 P2(1)/c homeotypes of dufrenoysite, 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.