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

Jacutingaite, Pt 2 HgSe 3 , 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 μm 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 VHN 10 (15 indentations on five grains of synthetic Pt 2 HgSe 3 ) are between 119 and 245 kg/mm 2 , with a mean value of 169 kg/mm 2 , corresponding to a Mohs hardness of approximately 3½. 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 (Pt 1.46 Pd 0.42 Cu 0.10 Ag 0.01 ) ∑1.99 Hg 0.98 Se 3.04 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 Pt 2.00 Hg 1.00 Se 3.00 . The mineral is trigonal, space group P 3 m 1, with a 7.3477(2), c 5.2955(1) A, V 247.59(1) A 3 and Z = 2. The crystal structure was solved and refined from the powder X-ray-diffraction data on synthetic Pt 2 HgSe 3 . Jacutingaite is isostructural with Pt 4 Tl 2 X 6 ( 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 A(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.