Montbrayite

 ( Sulfide Minerals ) 

The mineral formula was initially defined as Au₂Te₃ or , but more accurate analyses revealed that in different cases constant impurities are part of the montbrayite. Several formula variants are fixed in the calculated form, in particular, one of the variants concerns samples from the Robb Montbray type deposit and looks like (Au_1.73Bi_0.10Sb_0.06Pb_0.06Ag_0.04)_Σ=1.99Te_3.00.

Where montbrayite does not contain tellurobismuthite or altaite inclusions, its structure is generally homogeneous. Bireflection is barely noticeable. Anisotropy is weak to moderate, shades: light gray, light yellow-brown, bluish-gray.

Very similar minerals: calaverite, krennerite and montbrayite are difficult to distinguish under a microscope. A good way to determine this is by testing for microhardness. Another feature: montbrayite is characterized by a mosaic structure. In addition, montbrayite sometimes forms twins. When exposed to Nitric acid (in a 1:1 dilution), a kind of etching occurs, it boils strongly, bubbles form and yellow-brown spots of released gold remain on the surface of the mineral. Nina Sindeeva. Mineralogy, types of deposits and main features of geochemistry of selenium and tellurium. USSR Academy of Sciences. Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements. — Moscow: Publishing house Acad. Sciences of the USSR, 1959. — 257 p. With more saturated nitric acid (dilution 3:2) the reaction is less violent, the stain is light brown and small round halos are formed, also colored light brown; with concentrated nitric acid it boils weakly, and the surface is colored uniformly grayish-brown; round halos are not observed. Other reagents (HCl, KCN, FeCl₃, KOH and HgCl) do not affect the mineral.

Calaverite and montbrayite are among the earliest tellurides to form; they have relatively high microindentation hardness values, increased relief, and a pronounced tendency toward idiomorphism and the formation of coarse-crystalline varieties. The reflectivity of montbrayite is noticeably higher than that of calaverite; R_g of montbrayite almost completely repeats the R_ср curve of krennerite. In terms of reflectivity values in yellow light, relief, microindentation hardness, and the forms of separation in the form of short-prismatic plates, montbrayite and calaverite are similar to melonite, differing in the presence of a pronounced pinkish tint and more perfect cleavage in melonite.

Unlike hessite, sylvanite and other first-row minerals, empressite (AgTe), montbrayite (Au₂Te₃) and muthmannite (AuAgTe₂) are among the rare gold-silver tellurides. O. Yu. Plotinskaya. Deposits of Precious Metals. Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry of the Russian Academy of Sciences, Moscow. ― Miass: I-Min Ural Branch RAS, 2014. (in Russian)

In the Robb-Montbray mine, montbrayite was found in association with native gold, tellurobismuthite, altaite, petzite, melonite, chalcopyrite, pyrite, sphalerite, chalcocite and marcasite. The sizes of the segregations of the corresponding gold tellurides were sometimes very large and reached 1 cm in diameter, and montbrayite was either at contacts with other gold tellurides or had the form of an inclusion in them. In places, altaite with small admixtures of gold and petzite in the form of thin fibers crossed montbrayite. Sometimes tellurobismuthite with a small amount of altaite and petzite formed rounded inclusions in montbrayite. In the latter case, a eutectoid structure was present, apparently resulting from the decomposition of tellurides. Thompson R. M. (1949). Montbrayite, a new data. — Amer. Mineralogist, vol.34, No.5, p.345-346. Melonite crystals, sometimes rimmed with petzite, have also been found in montbrayite.