Tetrahedrite-(Hg), a new ‘old’ member of the tetrahedrite group

Abstract

Tetrahedrite-(Hg), Cu 6 (Cu 4 Hg 2 )Sb 4 S 13 , has been approved as a new mineral species using samples from Buca della Vena mine (hereafter BdV), Italy, Jedová hora (Jh), Czech Republic and Rožňava (R), Slovakia. It occurs as anhedral grains or as tetrahedral crystals, black in colour, with metallic lustre. At BdV it is associated with cinnabar and chalcostibite in dolomite veins. At Jh, tetrahedrite-(Hg) is associated with baryte and chalcopyrite in quartz–siderite–dolomite veins; at R it is associated with quartz in siderite–quartz veins. Tetrahedrite-(Hg) is isotropic, greyish-white in colour, with creamy tints. Minimum and maximum reflectance data for Commission on Ore Mineralogy wavelengths in air (BdV sample), R in %) are 32.5 at 420 nm; 32.9 at 546 nm; 33.2 at 589 nm; and 30.9 at 650 nm. Chemical formulae of the samples studied, recalculated on the basis of 4 (As + Sb + Bi) atoms per formula unit, are: (Cu 9.44 Ag 0.07 ) Σ9.51 (Hg 1.64 Zn 0.36 Fe 0.06 ) Σ2.06 Sb 4 (S 12.69 Se 0.01 ) Σ12.70 (BdV), Cu 9.69 (Hg 1.75 Fe 0.25 Zn 0.06 ) Σ2.06 (Sb 3.94 As 0.06 )S 12.87 (Jh) and (Cu 9.76 Ag 0.04 ) Σ9.80 (Hg 1.83 Fe 0.15 Zn 0.10 ) Σ2.08 (Sb 3.17 As 0.58 Bi 0.25 )S 13.01 (R). Tetrahedrite-(Hg) is cubic, I $\overline 4 $ 3 m , with a = 10.5057(8) Å, V = 1159.5(3) Å 3 and Z = 2 (BdV). Unit-cell parameters for the other two samples are a = 10.4939(1) Å and V = 1155.61(5) Å 3 (Jh) and a = 10.4725(1) Å and V = 1148.55(6) Å 3 (R). The crystal structure of tetrahedrite-(Hg) has been refined by single-crystal X-ray diffraction data to a final R 1 = 0.019 on the basis of 335 reflections with F o > 4σ( F o ) and 20 refined parameters. Tetrahedrite-(Hg) is isotypic with other members of the tetrahedrite group. Mercury is hosted at the tetrahedrally coordinated M (1) site, along with minor Zn and Fe. The occurrence of Hg at this position agrees both with the relatively large M (1)–S(1) bond distance (2.393 Å) and the refined site scattering. Previous occurrences of Hg-rich tetrahedrite and tetrahedrite-(Hg) are reviewed, and its relations with other Hg sulfosalts are discussed.