Dulanggouite, Bi6Te3 (Bi2Te), a new tetradymite-group mineral from Dulanggou gold mine, Sichuan Province, China

Abstract

Dulanggouite (IMA2024-067), ideally Bi6Te3, is a new member of the tetradymite group discovered in the Dulanggou gold deposit (E 101°36′30′′, N 30°58′50′′), Sichuan Province, China. It occurs as anhedral grains (up to 200 μm) associated with native gold, native bismuth, tsumoite, pilsenite, hedleyite, and ehrigite within quartz veins or at the margins of sulfides (pyrrhotite and pyrite). The mineral is cream-white with a metallic luster, flexible with an uneven fracture, and has a calculated density of 8.827 g/cm3. Electron microprobe analyses (EPMA) yield an empirical formula of Bi6.08Te2.91Se0.01 based on 9 atoms per formula unit. Single-crystal X-ray diffraction (SC-XRD) analysis determined the crystal structure of dulanggouite to be trigonal, space group P-3m1 (#164), with unit cell parameters a = 4.4639(4) Å, c = 17.9040(15) Å, V = 308.97(6) Å3 and Z = 1, refined to a final R1 of 0.0239 for 2437 (365 unique) reflections. The SC-XRD model resolves a characteristic 9-atom layer stacking sequence (Bi-Bi-Te-Bi-Te-Bi-Te-Bi-Bi) along the c-axis, exhibiting refined mixed Bi-Te occupancies at two specific sites. Atomic-resolution probe-corrected dark-field scanning transmission electron microscopy (DF-STEM) imaging and energy-dispersive X-ray spectrometry (EDS) mapping along the [11¯00] zone axis also confirms this 9-atom layer structure but does not support the proposed mixed occupancies. Dulanggouite represents the N = 9 member of the tetradymite homologous series. Within the Dulanggou deposit, native gold exhibits intimate microtextural intergrowths with dulanggouite, related Bi-rich tetradymite-group minerals (hedleyite and ehrigite), and native bismuth.