Lead-antimony sulfosalts from Tuscany (Italy). XXI. Bernarlottiite, Pb₁₂(As₁₀Sb₆)_ΣS16S₃₆, a new N = 3.5 member of the sartorite homologous series from the Ceragiola marble quarry: occurrence and crystal structure

The new mineral species bernarlottiite, Pb₁₂(As₁₀Sb₆)_Σ16S₃₆, has been discovered in cavities of the Early Jurassic marbles from the Ceragiola quarry, Seravezza, Apuan Alps, Tuscany, Italy. Its name honours Bernardino Lotti (1847–1933) for his significant contribution to the knowledge of the geology of Tuscany and to the development of the Tuscan mining industry. It occurs as lead-grey acicular crystals up to 1 mm in length and few μm in width, with a metallic lustre, associated with Sb-rich sartorite. Under the ore microscope, bernarlottiite is white with abundant red internal reflections; pleochroism is weak in air, with shades of grey-blue. Anisotropism is distinct to strong, with greyish-bluish rotation tints. Reflectance percentages for the four COM wavelengths are [R_min, R_max (%), (λ)]: 30.0, 37.5 (470 nm); 30.3, 37.3 (546 nm); 29.7, 36.8 (589 nm); and 29.3, 36.2 (650 nm). Electron-microprobe analyses, collected on two different grains, give (in wt%): Cu 0.09 (16), Pb 48.89 (1.26), As 17.48 (22), Sb 11.36 (10), S 23.11 (32), total 100.93 (1.38) (sample # 2987) and Pb 47.43 (26), As 14.56 (24), Sb 13.92 (18), S 22.64 (17), total 98.56 (44) (sample # 3819). On the basis of ΣMe = 28 atoms per formula unit, the chemical formulae are Cu_0.07(12)Pb_11.71(18)As_11.59(21)Sb_4.63(9)S_35.78(48) and Pb_11.92(6)As_10.12(14)Sb_5.95(8)S_36.76(32) for samples # 2987 and # 3819, respectively. The main diffraction lines, corresponding to multiple h k l indices, are [d in Å (relative visual intensity)]: 3.851 (s), 3.794 (s), 3.278 (s), 3.075 (s), 2.748 (vs), 2.363 (s), and 2.221 (vs). The crystal structure study gives a triclinic unit cell, space group P$1¯$⁠, with a = 23.704 (8), b = 8.386 (2), c = 23.501 (8) Å, α = 89.91 (1), β = 102.93 (1), γ = 89.88 (1), V = 4553 (2) ų, Z = 3. The crystal structure has been solved and refined to R₁ = 0.088 on the basis of 7317 reflections with F_o>4σ (F_o). Bernarlottiite is a new N = 3.5 homeotype of the sartorite homologous series, with a 3a superstructure relative to that of baumhauerite. Its crystal structure can be described as being formed by 1:1 alternation of sartorite-type (N = 3) and dufrénoysite-type (N = 4) layers along c, connected by Pb atoms with tricapped trigonal prismatic coordination. Each layer results from the stacking of two types of ribbons along a, a centrosymmetric one alternating with two acentric ones. The three main building operators of the structure are (1) the interlayers As-versus-Pb crossed substitution, stabilizing the combined N = (3, 4) baumhauerite homologue, (2) the inter-ribbon Sb partitioning in the sartorite-type layer, with “symmetrization” of the Sb-rich ribbon, that induces the 3a superstructure, and (3) the common (As, Sb) polymerization through short (As,Sb)–S bonds.