(Co, Fe)AsS, cobalt arsenic Sulfide
Important ore of cobalt and as mineral specimens
Cobaltite although rare is still an important and valuable ore of cobalt, a strategically and industrially useful metal. The symmetry of cobaltite is somewhat in dispute. Its structure is very similar to the structure of pyrite, FeS2. The sulfur to sulfur link (S-S) in pyrite is replaced by an arsenic to sulfur link (As-S) in cobaltite. If the position of the arsenic is not ordered then the symmetry is the same as pyrite’s symmetry which is in the isometric class, 2/m bar 3. However it appears from some x-ray spectroscopy studies that the arsenic is ordered there by breaking the higher symmetry and giving cobaltite a symmetry of the orthorhombic class, 2/m 2/m 2/m. But the debate is not settled yet.
Regardless of its actual symmetry, cobaltite forms isometric looking crystals. Either from really being isometric or from simply having such a similar structure to pyrite, cobaltite’s crystals mimic those of pyrite. Although the crystal habits are similar to pyrite, cobaltite can not be confused with pyrite which is brassy yellow in contrast to cobaltite’s silver gray or white colour. Skutterudite on the other hand is also white and forms similar crystals although it has poor cleavage.
Often deposits of cobaltite will have a weathering crust of minerals such as erythrite, Co3(AsO4)2-8(H2O). Since cobalt is a strong colouring metal, minerals like erythrite are strongly coloured, in this case a pink to bright purple. Miners called these colourful minerals “cobalt blooms” and used them as indicators of the presence of cobalt ores, such as cobaltite. Good crystals are usually common when cobaltite deposits are found and are a treasure for collectors.
Colour: white to silver gray
Transparency: crystals are opaque
Crystal System: it has been described as isometric; 2/m bar 3, but its actual structure is perhaps orthorhombic; m m 2 although the last word has not been said on this subject
Crystal Habits: include cubes, octahedrons, pyritohedrons and combinations of these isometric forms. If cobaltite is actually orthorhombic than these forms are either pseudocubes etc or they are pseudomorphs from a truly isometric phase which existed at higher temperature and/or pressure. Cobaltite is also commonly massive and granular
Cleavage: distinct in three directions forming cubes
Fracture: uneven to subconchoidal
Specific Gravity: approx. 6.0 – 6.4+ (heavier than average for metallic minerals)
Streak: dark gray
Other: Striations on cube faces
Associated Minerals: silver, chalcopyrite, pyrite, erythrite, skutterudite and other cobalt minerals
Major Occurrences: include cobalt, Ontario, Canada; Zaire; Siegerland, Germany; Skutterud, Norway; Tunaberg, Sweden; Sonora, Mexico; England; Boulder, Colorado and other USA localities
Best Indicators: crystal habit, cleavage, colour, streak, association with erythrite and luster