THE COMPOSITION AND FORMULA OF THE RARE GOLD TELLURIDE, MONTBRAYITE

Montbrayite is a rare, triclinic, gold-antimony telluride that is one of six known gold-bearing tellurides. The others are calaverite (AuTe₂), sylvanite ((Ag,Au)₂Te₄), krennerite ((Au,Ag)Te₂), petzite (Ag₃AuTe₂), and muthmannite ((Ag,Au)Te). Montbrayite is known from only five localities, the Robb-Montbray mine (Quebec), which is the type locality for this mineral, the Golden Mile deposit (Western Australia), Enasen (Sweden), Changjin (P.R.China), and Voronezhsky Massif (Russia). The optical properties of montbrayite vary from creamy-white to yellowish-white to pink in color. It is also rarely pleochroic, but shows weak to strong anisotropism. These optical properties are significantly altered by the trace element content. The IMA recognized formula for montbrayite is (Au,Sb)₂Te₃, which implies that Sb substitutes for Au in the structure of montbrayite. Previous studies have argued that Sb is essential to montbrayite because without Sb, montbrayite is metastable and will break down to calaverite and native gold. Montbrayite, up to 0.18 mm in length, coexists with altaite (PbTe) and petzite, native gold and petzite, and calaverite and native gold, and is intergrown with chalcopyrite, sphalerite, altaite, and pyrite in Fimiston-style mineralization in the Golden Mile. Electron microprobe compositions of montbrayite obtained herein from the Golden Mile reveal that it contains up to 7.46 wt. % Sb and is the most antimony-rich montbrayite yet reported. Published compositional data of montbrayite combined with the new data from the Golden Mile suggest that Sb and Bi substitute in both the Te and Au sites and that Sb is not essential to the stability of montbrayite if Bi is present. Bismuth appears to stabilize montbrayite in much the same manner as does Sb. This complex substitution of Bi and Sb for Au and Te suggests a modification of the formula of montbrayite to (Au,Sb)₂(Te,Sb,Bi)₃ or, more likely, (Au,Sb,Bi)₂(Te,Sb,Bi)₃.