CRYSTAL CHEMISTRY OF MERCURY SULFOSALTS – GALKHAITE, (Hg5+xCu1–x)Cs1–xAs4S12(x≈ 0): CRYSTAL STRUCTURE AND REVISION OF THE CHEMICAL FORMULA

Abstract

Abstract 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 [(Hg 4.61 Zn 0.38 Fe 0.04 ) Σ5.03 (Cu 1.03 Ag 0.04 ) Σ1.07 ] Σ6.10 (Cs 0.76 Tl 0.20 ) Σ0.96 (As 3.81 Sb 0.04 ) Σ3.85 S 12.06 (G), [(Hg 4.66 Zn 0.32 ) Σ4.98 (Cu 0.55 Ag 0.45 ) Σ1.00 ] Σ5.98 (Cs 0.74 Tl 0.06 ) Σ0.80 (As 3.42 Sb 0.56 ) Σ3.98 S 12.05 (Gk), and [(Hg 4.10 Zn 0.85 ) Σ4.95 (Cu 0.89 Ag 0.16 ) Σ1.05 ] Σ6.00 Cs 0.95 (As 3.64 Sb 0.16 ) Σ3.80 S 12.19 (S). Unit-cell parameters are a 10.405(3) A (G), 10.443(1) A (Gk), and 10.332(2) A (S), space group I 4 ¯ 3 m . 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 HgS 4 and AsS 3 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 Hg 2+ 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 (Hg 5 Cu)CsAs 4 S 12 . Taking into account the possible existence of vacancy in the structural cavities, the formula can be generalized as (Hg 5+ x Cu 1– x ) Σ6 Cs 1– x As 4 S 12 ( x ≈ 0).