Hedleyite
Chemical formula: Bi<sub>7</sub>Te<sub>3</sub>
Hedleyite is a rare bismuth telluride, forming flakes and grains with a metallic luster, found in quartz veins.
Properties
- Mohs hardness
- 2
- Luster
- Metallic
- Streak
- Greyish black
- Density
- 9.04
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Hedleyite can be identified by its silvery-white color with a pinkish tint, metallic luster, very high density, and perfect cleavage, which causes it to break into thin flakes. Its low hardness (2 on the Mohs scale) is also a key diagnostic feature. It occurs in a characteristic geological environment, in quartz veins with gold and other tellurides. ## Distinguishing from Similar Minerals Hedleyite can be confused with other bismuth tellurides, such as tellurobismuthite (Bi₂Te₃) or joseite. Distinguishing them based on visual characteristics is extremely difficult and often requires advanced chemical analyses (e.g., EDS) or X-ray analyses (XRD). It differs from graphite and molybdenite by its significantly higher density and silvery color (as opposed to gray or bluish). ## Crystal Forms Hedleyite crystallizes in the trigonal system. It most commonly forms irregular flakes, scales, and grains. Well-formed, tabular crystals with a hexagonal outline, usually small, not exceeding a few millimeters, are rarely observed.
Geological environment
## Genesis Hedleyite is a hydrothermal mineral. It forms in quartz-carbonate veins that cut contact metamorphic rocks, such as skarns. Its crystallization occurs under medium to high-temperature conditions, in an environment rich in bismuth and tellurium. ## Mineral Associations This mineral often co-occurs with native gold, as well as with other tellurides, such as tellurobismuthite, joseite, altaite, and hessite. It is also accompanied by sulfides, e.g., pyrite, chalcopyrite, and arsenopyrite. The rock-forming minerals in its vicinity are most often quartz, calcite, garnets, and pyroxenes. ## Localities The most important and classic hedleyite localities are in Canada, in the Hedley area of British Columbia (Good Hope and Nickel Plate mines). It is also known from the Boliden mine in Sweden, from several localities in Japan (Oya and Koganetsubo mines), and from Australia (Kalgoorlie mine in Western Australia).
Rarity
Rare
For collectors
## Quality Criteria For collectors, the most valuable specimens are those where hedleyite forms well-developed, macroscopically visible flakes or plates with a distinct metallic luster. Rich inclusions of hedleyite in a quartz matrix are also highly prized, especially when accompanied by visible native gold. Surface purity, lack of mechanical damage, and clear contrast with surrounding minerals increase the specimen's value. ## Popular Localities The most valued and sought-after specimens by collectors come from the type locality in British Columbia, Canada. Material from the Boliden mine in Sweden has also yielded good collector specimens.
Care and storage
## Cleaning Hedleyite specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to its softness and perfect cleavage, all friction and strong pressure should be avoided. Wet cleaning, especially with chemical cleaning agents, is not recommended. ## What to Avoid Avoid contact with acids and other chemicals that may react with tellurides. The mineral is soft and easily scratched, so it must be protected from contact with harder minerals. Prolonged exposure to moisture and air can cause superficial oxidation and tarnishing. ## Storage Hedleyite specimens are best stored in separate, padded boxes to prevent mechanical damage and scratches. They should be protected from dust and moisture. Due to its rarity, specimens should be handled with great care.