Daliranite, PbHgAs₂S₆, a new sulphosalt from the Zarshouran Au-As deposit, Takab region, Iran

Abstract

Daliranite, ideally PbHgAs₂S₆, occurs as a rare sulphosalt species at the Carlin-type Zarshouran Au-As deposit North of the town of Takab in the Province of West Azarbaijan, Iran. The new species is associated with orpiment, rarely with galkhaite, hutchinsonite and cinnabar. The strongly silicified matrix of the specimens has veinlets of sphalerite, with rare inclusions of galena and various (Cu)-Pb-As(Sb) sulphosalts. Daliranite occurs as matted nests of acicular and flexible fibres up to 200 mm in length and a width less than a few mm. The colour is orange-red with a pale orange-red streak and the lustre is adamantine. The mineral is transparent and does not fluoresce. The Mohs hardness is <2. Electron microprobe analyses give the empirical formula Pb0.95Tl0.01Hg1.04As2.10S5.91, ideally PbHgAs2S6; the calculated density is 5.93 g cm3. Unit-cell parameters were determined by an electron-diffraction study and refined from X-ray powder data. Daliranite is monoclinic primitive with a = 19.113(5) Â, b = 4.233(2) Â, c = 22.958(8) Â, b = 114.78(5)º, V = 1686.4 ³ and Z = 8, a:b:c = 4.515:1:5.424, space group P2, Pm or P2/m. The strongest X-ray powder-diffraction lines [d in Â, (I), (hkl)] are: 8.676, (80), (200); 4.654, (50), (401); 3.870, (40), (211); 3.394, (50), (113); 3.148, (40b), (602); 2.892, (50), (600); 2.724, (100), (703); 2.185, (50), (319). The formula shows a sulphur excess which may correspond to SS bonding (persulphide). The new sulphosalt is a late phase in the crystallization sequence, and was formed after orpiment, contemporaneously with quartz II, at a temperature between 157an d 193ºC. The name honours Dr Farahnaz Daliran (University of Karlsruhe, Germany) in recognition of her outstanding contributions to research on ore deposits, especially Au, Zn and Fe, in Iran.