THE COBALTITE GROUP OF MINERALS
The Cobaltite Group is composed of minerals with a similar isometric or pseudo-isometric structure and related chemistry. The general formula for this group is ABX. The A ion can be either cobalt, nickel, iridium, iron, palladium, platinum, rhodium and/or ruthenium. The B ion is usually a semi-metal such as antimony, arsenic, bismuth and/or tellurium. The X ion is usually sulfur, but can also be antimony, selenium and/or tellurium.
The structure of this group is similar to the structure of pyrite and other members of the Pyrite Group. The general formula for the Pyrite Group is AX2, with pyrite's formula being FeS2. The X2 chemical groups of the Pyrite Group are replaced by B-X chemical groups in the Cobaltite Group minerals. The semi-metals and usually sulfur are bonded together with a covalent bond. If the B-X chemical groups are not ordered, as in the B or the X are randomly placed with respect to each other, then the symmetry of the mineral would belong to the Diploidal Class; bar 3 2/m, a class that, although isometric, lacks any four fold symmetry. But if they are ordered then the symmetry is reduced to the lower symmetry of the Tetartoidal Class; 2 3. This is the lowest symmetry class of the isometric system and contains only three fold and two fold axes. Cobaltite may have even a lower symmetry of orthorhombic; m m 2. Only cobaltite is relatively common with most other members of this group being uncommon to rare.