THE NEW MINERAL SPECIES BRODTKORBITE, Cu2HgSe2, AND THE ASSOCIATED SELENIDE ASSEMBLAGE FROM TUMINICO, SIERRA DE CACHO, LA RIOJA, ARGENTINA

Abstract

Brodtkorbite, ideally Cu2HgSe2, a new mineral species, occurs in a telethermal selenide vein-type assemblage at the Tuminico Ia selenium deposit, Sierra de Cacho (Sierra de Umango) district, La Rioja, Argentina. The selenide assemblage also contains berzelianite, bellidoite, cadmoselite (mercurian, cuprian), chameanite, crookesite, eskebornite, eucairite, ferroselite, hakite, klockmannite, a luanheite-type phase, trogtalite-kruťaite, tiemannite, tyrrellite, umangite and uraninite. Minor constituents are undefined phases that belong to the Cd–(Cu)–Hg–Se and (Co,Ni,Cu)–As–Se systems. The selenides occur as veinlets, as impregnations and as massive ores in calcite veins, and are hosted by altered (hematite-stained) amphibolites of the Sierra de Pampeanas, the Precambrian basement of the Precordillera terrane. Brodtkorbite was observed as anhedral grains; they range in size from 10 × 20 up to 50 × 100 μm and as aggregates of composite grains up to 150 × 250 μm, commonly intergrown with berzelianite, tiemannite and (rarely) umangite and clausthalite. The mineral is dark grey, opaque, has a metallic luster, and a dark grey streak. VHN10 ranges from 91.4 to 131 (mean 118) kg/mm2, which corresponds to a calculated Mohs hardness of about 2½ to 3. The density is 7.77 g/cm3 and was calculated for the ideal formula with Z = 2. In plane-polarized reflected light, the mineral is white, is weakly to moderately pleochroic from pinkish to bluish white, and lacks internal reflections. It has a weak to strong bireflectance; the anisotropy is weak to strong with rotation tints from mauve-grey to yellowish grey. The reflectance spectra and color values in air and in oil are tabulated. The chemical composition, which was obtained with an electron microprobe, shows very little variation within individual grains and between grains. The average composition is: Cu 26.2, Hg 40.7, Se 32.9, total 99.8 wt%. This leads to an empirical formula (total atoms = 5) of Cu2.00Hg0.98Se2.02. Brodtkorbite is monoclinic, a 7.492(5), b 4.177(1), c 7.239(4) A, β 114.20(5)°, V 206.6(2) A3, a:b:c 1.7936:1:1.7330, space group P 21/ n (14). The strongest seven X-ray powder-diffraction lines [ d in A ( I ) ( hkl )] are: 3.991(70)(101), 3.576(50)(110), 3.534(50)(011), 3.414(50)(200), 2.731(100)(112), 2.223(70)(211) and 2.072(50)(113, 312). The mineral name honors Milka Kronegold de Brodtkorb, Professor at the universities of Buenos Aires and La Plata, Argentina, for her outstanding contributions to the mineralogy and economic geology of Argentina. The selenide mineralogy at Tuminico is discussed on the basis of an extensive electron-microprobe study. The conditions of formation for brodtkorbite and the associated selenides are inferred from mineral stabilities and fluid-inclusion studies.