Lead-antimony sulfosalts from Tuscany (Italy). XVII. Meerschautite, (Ag,Cu)_5.5Pb_42.4(Sb,As)_45.1S₁₁₂O_0.8, a new expanded derivative of owyheeite from the Pollone mine, Valdicastello Carducci: occurrence and crystal structure

Abstract

The new mineral species meerschautite, ideally (Ag,Cu)5.5Pb42.4(Sb,As)45.1S112O0.8, has been discovered in the baryte + pyrite ± (Pb-Zn-Ag) deposit of the Pollone mine, near Valdicastello Carducci, Apuan Alps, Tuscany, Italy. It occurs as black prismatic crystals, striated along [100], up to 2 mm long and 0.5 mm thick, associated with baryte, boulangerite, pyrite, quartz and sphalerite. Meerschautite is opaque with a metallic lustre and shows a black streak. In reflected light, meerschautite is white in colour, weakly bireflectant and non pleochroic. With crossed polars, it is distinctly anisotropic with grey to dark grey rotation tints with brownish and greenish shades. Reflectance percentages for COM wavelengths [λ (nm), Rair (%)] are: 470: 39.7/41.4; 546: 38.3/39.9; 589: 37.4/39.0; 650: 35.8/37.2. Electron-microprobe data collected on two different samples gave (wt.%): Cu 0.22, Ag 3.15, Tl 0.07, Pb 48.54, Sb 25.41, As 2.82, S 19.74, Se 0.14, Cl 0.03, sum 100.12 (# 1) and Cu 0.22, Ag 3.04, Tl 0.13, Pb 48.53, Sb 25.40, As 2.93, Bi 0.06, S 19.82, Se 0.13, Cl 0.05, sum 100.31 (# 2). On the basis of 112 anions (S+Se+Cl) per formula unit, the empirical formulae are (Ag5.29Cu0.63)∑5.92(Pb42.43Tl0.06)∑42.49(Sb37.80As6.82)∑44.62(S111.53Se0.32Cl0.15)∑112 (# 1) and (Ag5.08Cu0.62)∑5.70(Pb42.22Tl0.12)∑42.34(Sb37.61As7.07Bi0.05)∑44.73(S111.45Se0.30Cl0.25)∑112 (# 2). Main diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 3.762 (m), 3.663 (s), 3.334 (vs), 3.244 (s), 3.016 (m), 2.968 (m), 2.902 (m), 2.072 (ms). The crystal structure study gave a monoclinic unit cell, space group P21, with a = 8.2393(1), b = 43.6015(13), c = 28.3688(8) Å, β = 94.128(2)°, V = 10164.93(2) Å3, Z = 2. The crystal structure has been solved and refined to a final R1 = 0.122 on the basis of 49,037 observed reflections. The structure is based on two building blocks, both formed by a complex column with a pseudotrigonal Pb6S12 core and two arms of unequal lengths (short and long arms, respectively). Two different kinds of short arms occur in meerschautite. One is an Ag-rich arm, whereas the other shows localized Sb–O–Sb bonds. Meerschautite is an expanded derivative of owyheeite and has quasi-homeotypic relationships with sterryite and parasterryite.

Lead–Antimony Sulfosalts from Tuscany (Italy). XXIV. Crystal Structure of Thallium-Bearing Chovanite, TlPb26(Sb,As)31S72O, from the Monte Arsiccio Mine, Apuan Alps

“…These distances are slightly longer than those observed in chovanite from the Pollone mine [2], where As was the dominant cation at the Sb45 position. This kind of configuration is topologically identical to those observed for the O3 isolated oxygen in scainiite [15], in a synthetic (Pb,Mn,Sb) oxy-chloro-sulfide [16], in pillaite [17], pellouxite [18], and meerschautite [19].…”

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

“…Sb-rich analogues or varieties of arsenic sulfosalts were described, for example, from the Monte Arsiccio mine, i.e. boscardinite, the Tl-Sb homeotype of baumhauerite, and protochabournéite (Orlandi et al, 2012(Orlandi et al, , 2013.…”

“…The name is for the type locality, Valdicastello Carducci. This is the type locality of other two sulfosalts, parasterryite (Moëlo et al, 2011) and meerschautite (Biagioni et al, 2013). The name also remembers the family name Carducci: the Italian poet Giosué Carducci (1835Carducci ( -1907, Nobel Prize winner for literature in 1906, was born at Valdicastello.…”

“…During the last five years, a series of new or very rare Ag-Pb/Sb-As and Tl-Pb/Sb-As sulfosalts have been identified from the baryte-pyrite-iron oxide deposits of the southern Apuan Alps, Tuscany, Italy: sterryite, parasterryite, and meerschautite from the Pollone mine, near Valdicastello Carducci (Moëlo et al, 2011;Biagioni et al, 2013), boscardinite and protochabournéite from the Monte Arsiccio mine (Orlandi et al, 2012(Orlandi et al, , 2013.…”

Ciriottiite, Cu(Cu,Ag)3Pb19(Sb,As)22(As2)S56, the Cu-Analogue of Sterryite from the Tavagnasco Mining District, Piedmont, Italy

“…At its base, a mixed and split (Cu/Bi) site, namely Cu33, alternates with a mixed (Cu,Ag,Hg) site (Cu34); the former corresponds to a split (Cu/Ag) site in sterryite [8] (Figure 4b). At the end of the long arm, a (Cu,Hg) site (Cu25) alternates with a mixed (Sb,As) site (Sb26) (Figure 4c); in sterryite, as well as in parasterryite and meerschautite [8,11], a similar configuration with an (Ag,Cu) site alternating with an Sb one was reported [8].…”