Overview
Bixbyite is a rare manganese-iron oxide prized by collectors for its sharp, jet-black cubic crystals with a bright metallic sheen. Although the individual crystals are seldom larger than a centimetre, their clean geometric faces and high lustre make them strikingly photogenic, especially when perched on contrasting pink-orange topaz or pale rhyolite matrix. The species was described in 1897 from the Thomas Range in Utah and named for the prospector and mineral dealer Maynard Bixby, who first gathered specimens there. Under modern nomenclature the manganese-dominant species is formally called bixbyite-(Mn), reflecting that most material is far richer in manganese than iron.
Composition & structure
Bixbyite is a sesquioxide with the general formula (Mn,Fe)2O3, in which trivalent manganese and trivalent iron substitute freely for one another. The iron-to-manganese ratio is highly variable, and many crystals from the Utah type area contain very little iron at all. It crystallises in the cubic (isometric) system, building well-formed cubes, octahedra and dodecahedra with the same internal framework as the synthetic compound used in some catalysts. Bixbyite is the natural analogue of the manganese end of a series whose iron-dominant counterpart is a far rarer phase.
| Formula | (Mn,Fe)2O3 (manganese-dominant: bixbyite-(Mn)) |
| Crystal system | Cubic (isometric) |
| Mohs hardness | 6 to 6.5 |
| Lustre | Metallic to submetallic |
| Colour | Black to brownish-black |
| Type locality | Thomas Range, Juab County, Utah, USA |
Formation & occurrence
The classic Utah bixbyite forms in cavities and along fractures in topaz rhyolite, a volcanic rock rich in fluorine and incompatible elements. Here it crystallised from hot vapours circulating through the cooling lava, alongside the famous red beryl, topaz, pseudobrookite and hematite of the same flows. Bixbyite also occurs in quite different settings: in metamorphosed manganese ore bodies and skarns, where it grows during regional or contact metamorphism of manganese-bearing sediments. In these environments it associates with braunite, hausmannite, hematite and other manganese oxides rather than with volcanic minerals.
Identification & similar species
Bixbyite is recognised by its black colour, metallic lustre, isometric crystal habit and relatively high hardness of about 6 to 6.5, which distinguishes it from softer black manganese oxides such as pyrolusite. Its cubic symmetry and equant crystals separate it from the more tabular or prismatic habits of braunite and hausmannite, while its lack of magnetism distinguishes it from magnetite. On the Utah rhyolite, the diagnostic clue is its habit of forming isolated, brilliant black cubes sitting on or beside gem topaz and red beryl, an association found almost nowhere else.
Notable localities & collecting
The Thomas Range and the nearby Wah Wah Mountains in Utah remain the most celebrated source of crystallised bixbyite and a magnet for field collectors hunting topaz and red beryl. Outstanding masses and crystals also come from the manganese deposits of the Jhabua and Chhindwara districts in central India, an important occurrence for the species. Additional material is reported from localities in Sweden, Germany, Namibia, South Africa, Argentina and elsewhere. Because fine euhedral crystals are small and uncommon, well-formed Utah specimens on attractive matrix are sought after, and collectors should handle the lustrous faces carefully to avoid dulling them.