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

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 occur as anhedral grains, up to 0.5 mm in size, in the pyrite-rich dolostone and in the baryte + pyrite microcrystalline ore bodies from the Sant'Olga level. Associated minerals are baryte, boscardinite, cymrite, protochabourneite, 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 (Cu6.00Fe0.06)(Sigma 6.06)(Hg2.81Ag0.12Zn0.11Cd0.01)(Sigma 3.05)(AS(3.24)Sb(0.66))(Sigma 3.90)(S12.10Se0.03)(Sigma 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 R-1 = 0.0264 and 0.0285, respectively. Aktashite is trigonal, R3, with unit-cell parameters a 13.7752(14), c 9.3785(10) angstrom, V 1541.2(3) angstrom(3); laffittite is monoclinic, space group Cc, with unit-cell parameters a 6.6671(2), b 11.3334(3), c 7.7572(2) angstrom, 13 115.258(2), V 530.10(3) angstrom(3). Their crystal structures are threedimensional frameworks formed by Cu-, Ag-, and Hg-centred 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.