Langisite
Chemical formula: CoAs
Langisite is a rare cobalt arsenide with a metallic luster, forming small, hexagonal crystals, found in calcite veins.
Properties
- Mohs hardness
- 6-6.5
- Color
- Pink
- Luster
- Metallic
- Streak
- Black
- Density
- 0
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Hexagonal
Diagnostic features
## Identification Identifying langisite with the naked eye is practically impossible due to its small size. Identification requires advanced laboratory methods, such as reflectivity analysis in polarized light (characteristic anisotropy from reddish-brown to gray) and chemical analysis using an electron microprobe (EDS/WDS) to confirm the composition (cobalt and arsenic). ## Distinguishing from Similar Minerals Langisite can be confused with other cobalt and nickel arsenides, such as skutterudite, safflorite, or nickeline. It is distinguished from nickeline (NiAs) by the dominance of cobalt over nickel. From skutterudite (CoAs₃), it differs in stoichiometry (Co to As ratio) and crystallographic system (hexagonal vs. isometric). Definitive differentiation is only possible through chemical and crystallographic analysis. ## Crystal Forms It forms small, hexagonal crystals with a tabular or platy habit. It usually occurs as irregular grains intergrown with other minerals, forming inclusions in calcite or growing on safflorite crystals.
Geological environment
## Genesis Langisite is a hydrothermal mineral. It forms at relatively low temperatures in ore veins, mainly calcite-silver-cobalt veins. Its crystallization is associated with mineralization processes within sedimentary and volcanic rocks. ## Mineral Associations This mineral coexists with other arsenides, sulfides, and vein minerals. It is most commonly found in association with safflorite, skutterudite, cobaltite, rammelsbergite, nickeline, native silver, native bismuth, and calcite. ## Localities In addition to the type locality at the Langis mine in Ontario (Canada), its occurrence has been confirmed in several other places worldwide. These include deposits in the Bou Azzer region in Morocco, the Tunaberg mine in Sweden, and some deposits in Saxony, Germany. All these sites are classic localities for cobalt and nickel minerals.
Rarity
Very rare
For collectors
## Quality Criteria The quality of langisite specimens is primarily assessed based on the possibility of its identification and the abundance of its occurrence in the sample. Since the crystals are microscopic, specimens where langisite forms distinct, well-formed crystals or rich aggregates visible under magnification are most valued. Contrast with surrounding minerals (e.g., white calcite) also enhances the visual appeal of a "micromount" specimen. ## Popular Localities The most known and classic specimens come from the type locality – the Langis mine in Ontario, Canada. Research material also originates from the world-renowned cobalt deposits in the Bou Azzer region in Morocco.
Care and storage
## Cleaning Langisite specimens should generally not be cleaned mechanically or chemically due to their small size and tendency to oxidize. If necessary, compressed air can be used to remove dust. Wet cleaning is not recommended. ## What to Avoid Avoid contact with water, acids, and other chemicals that can accelerate the oxidation process and damage delicate crystals. The mineral is sensitive to moisture and atmospheric changes, which cause it to tarnish. It should not be heated. ## Storage Langisite specimens, being mostly microscopic, should be stored in airtight "micromount" containers that protect them from dust, moisture, and mechanical damage. Storage in a dry place is recommended.