Pratesiite, AgCuTe 2 : A new layered Ag–Cu intermetallic phase from the Good Hope mine, Vulcan, Colorado, U.S.A.

Pratesiite, AgCuTe 2 , is a new Cu-Ag-Te mineral species discovered in a fragment of the cameronite holotype from the Good Hope mine, Vulcan, Colorado, U.S.A., a classic Te-rich hydrothermal deposit characterized by complex Cu-Te and Pb-Te assemblages.The mineral occurs as extremely rare, subhedral to anhedral grains up to 12 µm across, closely associated with rickardite, vulcanite, cameronite, spiridonovite, and native tellurium.In reflected light, pratesiite is black with a black streak, metallic lustre, Mohs hardness ~3, VHN 100 = 135-150 kg•mm -2 , brittle fracture, and no observed cleavage.It shows moderate bireflectance, very weak pleochroism, weak anisotropy, and no internal reflections.Electron-microprobe analyses indicate a chemically simple composition with only Ag, Cu, and Te above detection limits; the empirical formula, based on 4 apfu, is Ag(Cu 0.68 Ag 0.29 ) 0.97 Te 2.03 , corresponding to the ideal formula AgCuTe 2 (d calc = 10.154g•cm -3 ).Single-crystal X-ray diffraction data show that pratesiite