Columbite
Columbite, also called niobite, niobite-tantalite and columbate, with a general chemical formula of (FeII,MnII)Nb2O6, is a black mineral group that is an ore of niobium. It has a submetallic luster, a high density, and is a niobate of iron and manganese. It forms a series with the tantalum-dominant analogue ferrotantalite and one with the manganese-dominant analogue manganocolumbite. The iron-rich member of the columbite group is ferrocolumbite, where some tin and tungsten may be present in the mineral. Yttrocolumbite is the yttrium-rich columbite with the formula (Y,U,Fe)(Nb,Ta)O
4, is a radioactive mineral found in Mozambique.
| Columbite | |
|---|---|
| General | |
| Category | Oxide minerals |
| Formula (repeating unit) | (FeII,MnII)Nb2O6, or in oxide formula (FeII,MnII)O·Nb2O5 |
| IMA symbol | Clb |
| Strunz classification | 4.DB.35 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pbcn |
| Identification | |
| Color | Black, brownish black. |
| Crystal habit | Massive – Granular – Common texture observed in granite and other igneous rock; Striated - Parallel lines on crystal surface or cleavage face. |
| Cleavage | [010] Distinct |
| Fracture | Sub Conchoidal: Fractures developed in brittle materials characterized by semi-curving surfaces. |
| Mohs scale hardness | 6 |
| Luster | Sub-metallic |
| Streak | Blackish brown |
| Specific gravity | 5.3–7.3, Average = 6.3 |
| Optical properties | Biaxial (+), b = 2.29–2.4 |
| Other characteristics | Radioactive, non-fluorescent. |
| References | |
Columbite has the same composition and crystal symmetry (orthorhombic) as tantalite. In fact, the two are often grouped together as a semi-singular mineral series called columbite-tantalite or coltan in many mineral guides. However, tantalite has a much greater specific gravity than columbite, more than 8.0 compared to columbite's 5.2.
Columbite is also very similar to tapiolite. These minerals have the same chemical composition but different crystal symmetry: orthorhombic for columbite and tetragonal for tapiolite. The largest documented single crystal of columbite consisted of plates 6 mm (0.24 in) thick measuring 76 cm × 61 cm (30 in × 24 in).
Columbite contains varying amounts of thorium and uranium, which makes it radioactive to various degrees.