CRYSTAL CHEMISTRY OF MERCURY SULFOSALTS - GALKHAITE, (Hg5+xCu1-x)Cs1-x As4S12(x ≈ 0): CRYSTAL STRUCTURE AND REVISION OF THE CHEMICAL FORMULA

Three specimens of galkhaite from the Getchell mine (G), Nevada, USA; the Gal-Khaya As-Hg-Sb deposit (Gk), Yakutia, Russia; and Signols (S), Piedmont, Italy, have been fully characterized through single-crystal X-ray diffraction and electron-microprobe analyses. Chemical data indicate the formulae [(Hg4.61Zn0.38Fe0.04)(Sigma 5.03)(Cu1.03Ag0.04)(Sigma 1.07)](Sigma 6.10)(Cs0.76Tl0.20)(Sigma 0.96) (As3.81Sb0.04)(Sigma 3.85)S-12.06 (G), [(Hg4.66Zn0.32)(Sigma 4.98)(Cu0.55Ag0.45)(Sigma 1.00)](Sigma 5.98)(Cs0.74Tl0.06)(Sigma 0.80)(As3.42Sb0.56)(Sigma 3.98)S-12.05 (Gk), and [(Hg4.10Zn0.85)(Sigma 4.95)(Cu0.89Ag0.16)Sigma(1.05)](Sigma 6.00)Cs-0.95(As3.64Sb0.16)(Sigma 3.80)S-12.19 (S). Unit-cell parameters are a 10.405(3) angstrom (G), 10.443(1) angstrom (Gk), and 10.332(2) angstrom (S), space group I (4) over bar 3m. The crystal structure of the three specimens has been solved to R-1 = 0.029 (G), 0.028 (Gk), and 0.031 (S). It is a three-dimensional framework of HgS4 and AsS3 polyhedra, with Cs hosted in large 12-fold coordinated cavities. The introduction of Cs+, as well as minor Tl+, at these structural positions requires the replacement of Hg2+ by Cu+. The mixed site occupancy at the Hg site of galkhaite is a case of valency-imposed double site-occupancy. Consequently, the idealized chemical formula of galkhaite can be written as (Hg5Cu) CsAs4S12. Taking into account the possible existence of vacancy in the structural cavities, the formula can be generalized as (Hg5+xCu1-x)(Sigma 6)Cs1-x As4S12 (x approximate to 0).