Montbrayite
Chemical formula: (Au,Ag,Sb,Bi,Pb)<sub>23</sub>(Te,Sb,Bi,Pb)<sub>38</sub>
Montbrayite is an extremely rare gold telluride, valued for its metallic luster and association with other rare minerals.
Properties
- Mohs hardness
- 2.5
- Luster
- Metallic
- Streak
- Gray-black
- Density
- 9.94
- Cleavage
- None
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Triclinic
Diagnostic features
## Identification Key diagnostic features of montbrayite include its very high density, low hardness (it can be scratched with a copper wire), metallic luster, and cream-yellow color on a fresh fracture. Conchoidal fracture and lack of cleavage are also characteristic. Its association with other tellurides and native gold is a strong indicator. ## Distinguishing from Similar Minerals Montbrayite can be confused with other rare gold tellurides, such as calaverite or krennerite. Distinguishing them based on visual characteristics is often impossible and requires advanced chemical analysis (e.g., EDS). It is distinguished from the much more common pyrite by its lower hardness, different color (pyrite is brass-yellow), and gray-black streak (pyrite's streak is greenish-black). ## Crystal Forms Montbrayite crystallizes in the triclinic system. It is almost always found in the form of anhedral grains and massive aggregates. Clearly developed crystals visible to the naked eye are extremely rare.
Geological environment
## Genesis Montbrayite is a hydrothermal mineral, forming at low temperatures. It occurs in quartz-gold veins rich in tellurium. ## Mineral Associations It most often co-occurs with other tellurides, such as altaite, petzite, coloradoite, calaverite, and krennerite. It is also accompanied by native gold, pyrite, chalcopyrite, sphalerite, and quartz. ## Localities The most important and historical locality is the Robb-Montbray mine in Quebec, Canada, from which the best-known specimens originate. This mineral has also been found in small quantities in the Kalgoorlie region of Western Australia and in the historic mining district of Săcărâmb (formerly Nagyág) in Romania.
Rarity
Very rare
For collectors
## Quality Criteria The collector's value of montbrayite primarily depends on its quantity in the host rock. Rich, massive inclusions with a distinct, creamy color are most highly prized. An additional advantage is the co-occurrence with clearly visible native gold or other rare tellurides. Since crystals practically do not occur, form is not a key criterion. ## Popular Localities Specimens from the type locality – the Robb-Montbray mine in Canada – are by far the most sought after by collectors. Material from this location is considered classic for this species and fetches the highest prices on the market.
Care and storage
## Cleaning Montbrayite is very soft and brittle, so it should be cleaned with the utmost care. The safest method is to use a soft brush to remove dust or compressed air. If necessary, the specimen can be rinsed with distilled water and immediately and thoroughly dried. ## What to Avoid Ultrasonic cleaners, all chemical agents, and especially acids, should be strictly avoided. The mineral easily tarnishes upon contact with air and moisture, so it should not be stored in humid conditions. It should be protected from impacts and contact with harder minerals that could scratch it. ## Storage It is recommended to store specimens in closed, airtight containers (e.g., perky boxes) to limit the oxidation and tarnishing process. Display should be in stable conditions, away from sources of contamination and moisture.