Mercury-arsenic sulfosalts from Apuan Alps (Tuscany, Italy). II. Aktashite, Cu6Hg3As4S12, and laffittite, AgHgAsS3, from the Monte Arsiccio mine: occurrence and crystal structure

Abstract

Aktashite, ideally Cu₆Hg₃As₄S₁₂, and laffittite, AgHgAsS₃, were identified on specimens from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy, on the basis of X-ray diffraction and chemical data. Aktashite and laffittite  occurs as anhedral grains, up to 0.5 mm in size, in the pyrite-rich dolostone and in the baryte+pyrite microcrystalline ore bodies in the Sant’Olga level. Associated minerals are baryte, boscardinite, cymrite, protochabournéite, pyrite, realgar, Hg-rich sphalerite, stibnite, and some still unidentified phases. Electron-microprobe data of aktashite point to the following composition (in wt%): Cu 22.53(28), Ag 0.75(16), Fe 0.20(12), Hg 33.28(41), Zn 0.43(2), Cd 0.07(6), As 14.33(22), Sb 4.71(30), S 22.93(9), Se 0.15(15), total 99.39(25). The structural formula, based on 13 cations per formula unit, is (Cu_6.00Fe_0.06)_Σ=6.06(Hg_2.81 Ag_0.12Zn_0.11Cd_0.01)_Σ3.05(As_3.24Sb_0.66)_Σ3.90(S_12.10Se_0.03)_Σ=12.13. Laffittite was only qualitatively analyzed, showing Hg, Ag, As, S, and minor Sb as the only elements with Z > 9. The crystal structures of aktashite and laffittite were refined to R₁ = 0.0264 and 0.0285, respectively. Aktashite is trigonal, R3, with unit-cell parameters a 13.7752(14), c 9.3785(10) Å, V 1541.2(3) ų; laffittite is monoclinic, space group Cc, with unit-cell parameters a 6.6671(2), b 11.3334(3), c 7.7572(2) Å, β 115.258(2)°, V 530.10(3) ų. Their crystal structures are three-dimensional frameworks formed by Cu-, Ag-, and Hg-centered tetrahedra, connected through corner-sharing with isolated AsS₃ trigonal pyramids. Both phases show Hg in four-fold coordination. The dissymmetry of tetrahedral Hg site in laffittite (2+2, i.e. two shorter and two longer bonds) relatively to aktashite (regular tetrahedral) is correlated to a similar dissymmetry of Ag versus Cu coordinations. Aktashite and laffittite increase the list of rare Hg sulfides and sulfosalts identified in the baryte-pyrite-iron oxides deposits from Apuan Alps.