# Montbrayite

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**Montbrayite** (from a Canadian toponym) is a very rare mineral from among the gold tellurides, close to krennerite and calaverite, in composition it is a mixed polymetallic plumbo-telluride of gold with a variable formula, initially written as Au2Te3,[4]: 20–21 or (Au,Sb)2Te3,[1] but today having a much more complex form in the calculated form: (Au,Ag,Sb,Bi,Pb)23(Te,Sb,Bi,Pb)38. The color of montbrayite is cream, tin-white to pale yellow, the luster is metallic.

## Discovery history and name

The mineral was first identified in 1946 by M. A. Peacock and R. M. Thompson at the Canadian Robb Montbray deposit ([Quebec](/source/Quebec)).[5] Almost immediately, it was analyzed and a description of montbrayite was published as a new gold telluride with the formula Au2Te3.[6] The mineral was named after the location of its discovery, the typical Canadian Montbray deposit,[3] which remained the only one for montbrayite for the next quarter of a century.[7]: 28

## Properties

Montbrayite forms small segregations, rarely exceeding 3–5 mm, the mineral is dense, usually homogeneous, very brittle, the fracture is flat-conchoidal, sometimes discontinuous separation is observed. The hardness of montbrayite is low, which is quite common for [tellurides](/source/Telluride_mineral), it is about 2.5 on the [Mohs scale](/source/Mohs_scale). The specific gravity is about 9.94. The luster is metallic, on chips it gradually oxidizes and becomes cloudy, the color is tin-white to pale yellow. The polished surface is creamy-white, similar to that of [krennerite](/source/Krennerite); but not as white as [altaite](/source/Altaite).[4]: 20–21

The mineral formula was initially defined as Au2Te3[4]: 20 or (Au,Sb)2Te3,[1] 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 (Au1.73Bi0.10Sb0.06Pb0.06Ag0.04)Σ=1.99Te3.00.[3]

Where montbrayite does not contain [tellurobismuthite](/source/Tellurobismuthite) or [altaite](/source/Altaite) inclusions, its structure is generally homogeneous. Bireflection is barely noticeable. [Anisotropy](/source/Anisotropy) is weak to moderate, shades: light gray, light yellow-brown, bluish-gray.[4]: 21

Very similar minerals: [calaverite](/source/Calaverite), [krennerite](/source/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 [HNO3](/source/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.[8]: 239 With more saturated [nitric acid](/source/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](/source/Hydrochloric_acid), [KCN](/source/Potassium_cyanide), [FeCl3](/source/Iron(III)_chloride), [KOH](/source/Potassium_hydroxide) and [HgCl](/source/Mercury(II)_chloride)) do not affect the mineral.[4]: 21–22

[Calaverite](/source/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; *Rg* of montbrayite almost completely repeats the *Rср* curve of [krennerite](/source/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](/source/Melonite), differing in the presence of a pronounced pinkish tint and more [perfect cleavage](/source/Cleavage_(crystal)) in melonite.[7]: 39

## Conditions of formation

Montbrayite is a typical mineral of tellurium-gold deposits, closely associated with [native gold](/source/Native_gold), [petzite](/source/Petzite), [altaite](/source/Altaite), [coloradoite](/source/Coloradoite), [melonite](/source/Melonite) and [tellurobismuthite](/source/Tellurobismuthite), forming rounded inclusions and veinlets in [tetrahedrite](/source/Tetrahedrite) and [chalcopyrite](/source/Chalcopyrite). The size of fine-grained aggregates of montbrayite, as a rule, does not exceed 2.5 mm.[9]: 111

Unlike [hessite](/source/Hessite), [sylvanite](/source/Sylvanite) and other first-row minerals, [empressite](/source/Empressite) (AgTe), montbrayite (Au2Te3) and [muthmannite](/source/Muthmannite) (AuAgTe2) are among the rare gold-silver tellurides.[10]: 106

In the Robb-Montbray mine, montbrayite was found in association with [native gold](/source/Native_gold), [tellurobismuthite](/source/Tellurobismuthite), [altaite](/source/Altaite), [petzite](/source/Petzite), [melonite](/source/Melonite), [chalcopyrite](/source/Chalcopyrite), [pyrite](/source/Pyrite), [sphalerite](/source/Sphalerite), [chalcocite](/source/Chalcocite) and [marcasite](/source/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](/source/Gold) and [petzite](/source/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.[11] Melonite crystals, sometimes rimmed with [petzite](/source/Petzite), have also been found in montbrayite.[4]: 22

## Mineral formation

For the first 25 years after the mineral's discovery, the Robb Montbray type deposit remained the only one for montbrayite, as well as for the [frohbergite](/source/Frohbergite) first found there.[7]: 28 Montbrayite was first identified in the USSR in 1979 in [Uzbekistan](/source/Uzbekistan) in the ores of the Kochbulak gold-polymetallic deposit. However, the optical properties of the Uzbek montbrayite differed significantly from the Canadian montbrayite described in 1946 by Peacock and Thompson.[12]: 293 The first samples were found in close intergrowth with altaite, where montbrayite formed as a *«shirt»* on segregations of [native gold](/source/Native_gold) of very high purity, 970. Morphologically, montbrayite was formed first, its thin strip bordered the gold segregation, and then larger segregations in intergrowth with altaite surrounded the entire gold segregation. In reflected light, the mineral had a pinkish-brown hue.[13]: 463

Later, montbrayite was also discovered in [Russia](/source/Russia), in the Kochkarskoye gold deposit ([Chelyabinsk region](/source/Chelyabinsk_Oblast), [Plastovsky district](/source/Plastovsky_District)). Several mines with identified montbrayite are also located in [Kazakhstan](/source/Kazakhstan) ([Akmola](/source/Akmola_Region) and [East Kazakhstan regions](/source/East_Kazakhstan_Region)).[14]

To date, there are about two dozen gold-telluride deposits around the world in which samples of montbrayite have been found.[1]

## See also

- [Tellurides](/source/Telluride_mineral)
- [Stützite](/source/St%C3%BCtzite)
- [Sylvanite](/source/Sylvanite)
- [Nagyágite](/source/Nagy%C3%A1gite)
- [Frohbergite](/source/Frohbergite)
- [Calaverite](/source/Calaverite)
- [Krennerite](/source/Krennerite)
- [Bezsmertnovite](/source/Bezsmertnovite)
- [Volynskite](/source/Volynskite)
- [Bilibinskite](/source/Bilibinskite)

## References

1. [Montbrayite](https://www.mindat.org/min-2761.html) (A valid IMA mineral species, grandfathered): information about the mineral *montbrayite* in the Mindat database.

1. [Montbrayite](https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Montbrayite) on the website Mineralienatlas *(in German)*

1. *John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols* (Hrsg.): Handbook of Mineralogy, Mineralogical Society of America. 2001. — [Montbrayite](https://www.handbookofmineralogy.org/pdfs/montbrayite.pdf).

1. *O. M. Shubnikova*. New mineral species and varieties discovered in 1945-1949. Proceedings of the Institute of Geological Sciences. Issue 144, mineralogical and geochemical series (No. 16). — Moscow: USSR Academy of Sciences Publishing House, 1953. — 155 p. *(in Russian)*

1. Robb-Montbray Mine, Rouyn-Noranda TE, Abitibi-Témiscamingue, [Québec](/source/Quebec), [Canada](/source/Canada).

1. *Peacock M. A., Thompson R. M.* (1946). Montbrayite, a new gold telluride. — Amer. Mineralogist, vol.31, No.3-4, p.515-526.

1. *[Bezsmertnaya M. S.](/source/Marianna_Bezsmertnaya), Logikova L. A., Soboleva L. N.* Determination of tellurides under a microscope. — Moscow: Nauka, 1969. — 175 p. *(in Russian)*

1. *[Nina Sindeeva](https://ru.wikipedia.org/wiki/%D0%A1%D0%B8%D0%BD%D0%B4%D0%B5%D0%B5%D0%B2%D0%B0%2C_%D0%9D%D0%B8%D0%BD%D0%B0_%D0%94%D0%BC%D0%B8%D1%82%D1%80%D0%B8%D0%B5%D0%B2%D0%BD%D0%B0)*. Mineralogy, types of deposits and main features of geochemistry of [selenium](/source/Selenium) and [tellurium](/source/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.

1. *[Nekrasov I. Ya.](https://ru.wikipedia.org/wiki/%D0%9D%D0%B5%D0%BA%D1%80%D0%B0%D1%81%D0%BE%D0%B2%2C_%D0%98%D0%B2%D0%B0%D0%BD_%D0%AF%D0%BA%D0%BE%D0%B2%D0%BB%D0%B5%D0%B2%D0%B8%D1%87)* Geochemistry, mineralogy and genesis of gold deposits. — Moscow: Nauka, 1991. — 302 p. *(in Russian)*

1. *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)*

1. *Thompson R. M.* (1949). Montbrayite, a new data. — Amer. Mineralogist, vol.34, No.5, p.345-346.

1. *Genkin A. D., Safonov Yu. G., Tsepin A. I., Shcherbachev D. K.* Montbreyite from the Voronezh massif. — Moscow: Notes of the All-Russian Mineralogical Society, Volume 128, Issues 1-3. — St. Petersburg: Nauka, 1999. *(in Russian)*

1. *Ryabova E. G., Badalova R. P., Dubakina L. S.* Montbreyite — the first find in the USSR. — Moscow: Reports of the USSR Academy of Sciences. Volume 246, issues 2-3. 1979. *(in Russian)*

1. [Montbrayite (Монтбрейит)](https://webmineral.ru/minerals/item.php?id=21980) in the webmineral.ru database: minerals and deposits of Russia *(in Russian)*

## Publications

- *Palache, Charles, Harry Berman & Clifford Frondel* (1944), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837–1892, Volume I: Elements, Sulfides, Sulfosalts, Oxides. John Wiley and Sons, Inc., New York. 7th edition, revised and enlarged, 834 pp.: 260.
- *Thompson, R.M., M.A. Peacock, J.F. Rowland*, and *L.G. Berry* (1951) Empressite and “stuetzite”. Amer. Mineral., 36, 458–469.
- *Honea, R.M.* (1964) Empressite and stuetzite redefined. Amer. Mineral., 49, 325–338.
- *Cabri, L.J.* (1965) Discussion of “empressite and stuetzite redefined” by R.M. Honea. Amer. Mineral., 50, 795–801.
- *Stumpfl, E.F. and J. Rucklidge* (1968) New data on natural phases in the system Ag–Te. Amer. Mineral., 53, 1513–1522.
- *[Bezsmertnaya M. S.](/source/Marianna_Bezsmertnaya), Logikova L. A., Soboleva L. N.* (1969) Determination of tellurides under a microscope. — Moscow: Nauka. — 175 p. *(in Russian)*
- *Criddle, A.J.* and *C.J. Stanley,* Editors. (1993) Quantitative data file for ore minerals, 3rd ed. Chapman & Hall, London, 154.
- *Bindi, L., P.G. Spry*, and *C. Cipriani* (2004) Empressite, AgTe, from the Empress-Josephine mine, Colorado, U.S.A.: composition, physical properties, and determination of the crystal structure. American Mineralogist (2004): 89: 1043–1047
- *Vikent’eva, O.V., Shilovskikh, V.V., Shcherbakov, V.D., Moroz, T.N., Vikentyev, I.V., Bortnikov N.S*. (2023): Montbrayite from the Svetlinsk Gold–Telluride Deposit (South Urals, Russia): Composition Variability and Decomposition. Minerals, 13, 1225.

## External links

- [Montbrayite](https://www.mindat.org/min-2761.html) (A valid IMA mineral species, grandfathered): information about the mineral *montbrayite* in the Mindat database.
- [Montbrayite](https://www.handbookofmineralogy.org/pdfs/montbrayite.pdf): page in Handbook of mineralogy.
- [Montbrayite (Монтбрейит)](https://wiki.web.ru/wiki/Монтбрейит): Geowikipedia page *(in Russian)*.
- [Montbrayite (Монтбрейит)](https://catalogmineralov.ru/mineral/montbrayite.html): on the website catalogmineralov.ru *(in Russian)*.
- [Montbrayite (Монтбрейит)](https://webmineral.ru/minerals/item.php?id=21980) in the webmineral.ru database: minerals and deposits of Russia *(in Russian)*
- [Montbrayite](https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Montbrayite) on the website Mineralienatlas *(in German)*

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Adapted from the Wikipedia article [Montbrayite](https://en.wikipedia.org/wiki/Montbrayite) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Montbrayite?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.