Hastite
Chemical formula: CoSe<sub>2</sub>
Hastite is a very rare cobalt selenide with a metallic luster, forming tiny, cubic crystals.
Properties
- Mohs hardness
- 5.5
- Luster
- Metallic
- Streak
- Black
- Density
- 8.00
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Cubic
Diagnostic features
## Identification Identification of hastite is impossible with the naked eye and requires advanced laboratory techniques. Under a reflected light ore microscope, it appears white with a creamy tint. Final confirmation requires chemical analysis (e.g., electron microprobe - EDS/WDS) to determine the presence of cobalt and selenium in appropriate proportions. ## Distinguishing from similar minerals It is distinguished from other selenides and sulfides with a metallic luster (e.g., pyrite, cobaltite, clausthalite) based on its specific chemical composition (combination of cobalt and selenium). Optical properties in reflected light are crucial for initial differentiation from co-occurring minerals. ## Crystal forms Hastite crystallizes in the isometric system, forming very small, well-formed crystals with a cubic habit, less commonly octahedral. It typically occurs as single, dispersed inclusion crystals.
Geological environment
## Genesis Hastite is a hydrothermal mineral. It forms at low temperatures in polymetallic deposits rich in selenium. In its type locality (Öster Silvberg deposit), it is associated with calcite veins cutting through dolomites. ## Mineral associations This mineral occurs in association with other, often rare, selenides and sulfides. It most commonly co-occurs with clausthalite, cobaltite, pyrite, chalcopyrite, sphalerite, galena, as well as with freboldite and trogtalite. ## Localities It is an extremely rare mineral, known from only a few localities worldwide. Besides the type locality in Sweden (Hast mine, Öster Silvberg), its occurrence has been confirmed in Argentina (Sierra de Cacho, La Rioja province) and Germany (Trogtal mine, Harz mountains).
Rarity
Extremely rare
For collectors
## Quality criteria The quality of a hastite specimen is assessed quite differently than for common minerals. The most important criterion is the confirmation of the mineral's presence itself. Most valued are specimens where hastite crystals, despite their microscopic size, are well-formed and visible against a contrasting matrix. The richness of association with other rare selenides also significantly increases the scientific and collector value of the specimen. ## Popular localities The most "classic" and sought-after specimens, though still extremely rare, come from the type locality in Sweden (Öster Silvberg).
Care and storage
## Cleaning Due to the microscopic size of the crystals and their occurrence as inclusions in the host rock, cleaning individual hastite specimens is not practical. The host specimen can be cleaned with compressed air to remove dust. ## What to avoid Avoid contact with aggressive chemicals, especially acids, which can react with associated minerals. Do not subject the specimen to ultrasound or sudden temperature changes. Due to its selenium content, avoid inhaling dust during any potential processing. ## Storage Specimens containing hastite should be stored in stable conditions, in a dry place, away from direct sunlight. It is best to store them in closed collector boxes (so-called micromounts) to protect them from dust and mechanical damage.