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

• DOI: 10.2113/gscanmin.40.1.225
• Publication Date: 2007-07-19T21:56:23Z
• Authors: W. H. Paar; D. Topa; A. C. Roberts; A. J. Criddle; G. Amann; R. J. Sureda
• Keywords: Assemblage (archaeology); Selenide

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