Alloclasite

Alloclasite is a rare cobalt arsenic sulfide mineral, CoAsS. It is scientifically notable as a dimorph of cobaltite, meaning it has the same chemical composition but a different crystal structure.

🌍 Origins and Naming

Name Origin: From Greek allos (“other”) and klasis (“fracture”), referring to its distinct cleavage compared to cobaltite.
Discovery: First described in 1870.
Type Locality: Långban, Värmland, Sweden — a famous locality for rare manganese and arsenic minerals.

🔬 Chemical and Structural Properties

Formula: CoAsS
Mineral Group: Sulfides (arsenides).
Crystal System: Monoclinic (dimorph of cobaltite, which is cubic).
Color: Steel-gray to black.
Habit: Massive, granular; crystals are rare.
Hardness: ~5 on Mohs scale.
Specific Gravity: ~6.1–6.3 (dense due to cobalt and arsenic).
Luster: Metallic.
Streak: Black.

⚙️ Geological Occurrence

Found in hydrothermal veins and metamorphosed ore deposits.
Associated Minerals: Cobaltite, arsenopyrite, skutterudite, safflorite, and other Co–Ni–As sulfides.
Localities:
- Långban, Sweden (type locality).
- Other occurrences in Austria, Germany, and Slovakia.
- Rarely found in cobalt-rich hydrothermal deposits worldwide.

📖 Scientific and Collector Significance

Petrology: Important for understanding polymorphism in sulfide minerals (alloclasite vs. cobaltite).
Collectors: Rare, usually unattractive massive specimens, valued for rarity and mineralogical interest.
Economic Note: Contains cobalt, but too rare to be an ore mineral.

⚠️ Safety Considerations

Contains arsenic, so specimens should be handled carefully — avoid inhaling dust or ingesting particles, and wash hands after handling.

✨ Conclusion

Alloclasite is a rare cobalt arsenic sulfide (CoAsS), first described from Långban, Sweden. It is scientifically significant as the monoclinic dimorph of cobaltite, though it remains a collector’s curiosity rather than an economic resource.

In short: Alloclasite = CoAsS, cobalt arsenic sulfide, dimorph of cobaltite, rare, first found at Långban, Sweden.