JACUTINGAITE, Pt2HgSe3, A NEW PLATINUM-GROUP MINERAL SPECIES FROM THE CAUE IRON-ORE DEPOSIT, ITABIRA DISTRICT, MINAS GERAIS, BRAZIL

Jacutingaite, Pt2HgSe3, is a new species of platinum-group mineral discovered at the Caue iron-ore deposit, Itabira district, Minas Gerais, Brazil. Observed in a polished section, the mineral is about 50 mu m across and occurs on an aggregate of atheneite, potarite and hematite. The new mineral is partly altered to a Pt-O phase. Synthetic jacutingaite is megascopically grey in color, has a metallic luster and a grey streak. The mineral is brittle, with a very good {001} cleavage. Values of VHN10 (15 indentations on five grains of synthetic Pt2HgSe3) are between 119 and 245 kg/mm(2), with a mean value of 169 kg/mm(2), corresponding to a Mohs hardness of approximately 31/2. Under plane-polarized light, jacutingaite is light grey, has a moderate to distinct bireflectance, a bluish grey to rusty brown pleochroism, and a weak to distinct anisotropy; it does not exhibit internal reflections. Reflectance values of synthetic jacutingaite in air (R-max, R-min, in %) are: 51.1, 47.4 at 470 nm, 50.5, 48.2 at 546 nm, 49.6, 48.0 at 589 nm, and 47.8, 47.1 at 650 nm. Three electron-microprobe analyses of natural jacutingaite gave an average composition: Pt 37.30, Pd 5.91, Hg 25.72, Ag 0.16, Cu 0.82, and Se 31.48, total 101.39 wt.%, corresponding to the empirical formula (Pt1.46Pd0.42Cu0.10Ag0.01)(Sigma 1.99)Hg0.98Se3.04 based on six atoms; the average result of nine analyses of synthetic jacutingaite is: Pt 47.14, Hg 24.24, and Se 28.62, for a total of 100.00 wt.%, corresponding to Pt2.00Hg1.00Se3.00. The mineral is trigonal, space group P (3) over bar m1, with a 7.3477(2), c 5.2955(1) angstrom, V 247.59(1) angstrom(3) and Z = 2. The crystal structure was solved and refined from the powder X-ray-diffraction data on synthetic Pt2HgSe3. Jacutingaite is isostructural with Pt4Tl2X6 (X = S, Se, or Te); no structural analogue is known as a mineral. Jacutingaite is structurally related to sudovikovite. The strongest lines in the X-ray powder-diffraction pattern of synthetic jacutingaite [d in angstrom(I)hkl] are: 5.2917(100)001, 2.7273(16)201, 2.4443(10)012, 2.0349(18)022, 1.7653(37)003, 1.3240(11)004 and 1.0449(11)025. The mineral is named after the specular-hematite-rich vein-type mineralization locally known as "jacutinga", in which it occurs.