Marrucciite, a rare Hg-sulfosalt from the Gelnica ore deposit (Slovak Republic), and its comparison with the type occurrence at Buca della Vena mine (Italy)

The rare Hg-sulfosalt, marrucciite, with ideal formula Hg3Pb16Sb18S46, was found in the Gelnica ore deposit (Spi sko-Gemerské Rudohorie Mts., Slovak Republic). It occurs as acicular crystals up to 0.5 mm in length and fine-grained aggregates up to 1 mm in quartz (+siderite/dolomite) gangue in association with other sulfosalts (zinkenite, scainiite, boulangerite, chalcostibite, bournonite) and cinnabar. It is opaque, dark steel grey to grayish black, sometimes tarnished to a bronze-brown tint, has a dark grey streak with a very weak reddish tint and a metallic luster. Marrucciite is very brittle but thin acicular crystals are flexible. One good cleavage parallel to the elongation of the crystals was observed. The VHN20g microhardness is 85 (69-101) kp/mm2 and corresponds to a Moh’s hardness of c. 2; the calculated density is 6.00 g.cm-3. In reflected light, marrucciite is white to grayish white with a weak yellowish tint, distinct bireflectance and white-white grey pleochroism. Under crossed polars, it has distinct anisotropy, with rotation tints in shades of grayish brown and dark brown. Dark red internal reflections were observed only rarely. Marrucciite is monoclinic, space group C2/m with a 48.12(1), b 4.1083(2), c 23.990(5) , β 118.76(2)o, V 4158.(3) 3. Its average composition (electron microprobe data in wt. %) is Cu 0.03, Hg 7.83, Cd 0.02, Fe 0.06, Zn 0.02, Pb 43.13, Sn 0.03, Sb 29.15, Bi 0.12, S 19.71, Cl 0.05, total 100.15. The resulting empirical formula on the basis 83 of apfu is Hg2.93(Pb15.63Fe0.08Cu0.04Cd0.02Zn0.02Sn0.02)Σ15.81 (Sb17.97Bi0.04)Σ18.01(S46.14Cl0.11)Σ46.25. The crystal structure of marrucciite from Gelnica contains two Hg positions (6 atoms), 7 Pb positions (28 atoms), 8 Sb positions (32 atoms), two mixed Pb-Sb positions (refined as Pb0.39Sb0.61 and Pb0.61Sb0.39) and 23 S positions (46 atoms).