COLUMBITE
Columbite – (Mg,Fe++,Mn)(Nb,Ta)2O6 – is a rare oxide. It is the most common Nb ore, as well as being a Ta ore.
“Columbite”, by the current classification, is no longer a mineral species, but only a generic term used for members of the Columbite Group, composed of the Columbite-(Fe)-Tantalite-(Fe) Series and the Columbite-(Fe)-Columbite-(Mn) Series. Among the columbites, columbite-(Fe) ((Fe++Nb2O6)) is the most common. There is a variety with Sc.
Columbite tends to occur in well-formed crystals, which can form parallel, subparallel to radial clusters with a total weight of up to 18 tons. The largest columbite crystal was a plate 6 mm thick with sides of 76 cm by 61 cm.
Sometimes columbite is weakly magnetic (it is paramagnetic) and will be radioactive when it has uraninite inclusions. Therefore, it is important to measure the radioactivity of ores containing columbite.
Since columbites are macroscopically indistinguishable from tantalites, the collective term “COLTAN” is used for both minerals. And several books treat the two minerals together, as if they were one. The biggest difference between columbite and tantalite is density: columbite has a density of 5.2 and tantalite has a density greater than 8.0. In addition, columbites are very similar to wolframite.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Black, blackish-brown, often with an iridescent coating.
Habit: Massive, granular, short prismatic, equigranular, tabular.
Cleavage: {100} distinct, {010} less distinct. Striations parallel to (001).
Tenacity: Brittle.
Twinning: Twinning is common, can be contact or interpenetrating, heart-shaped, by {021}, {023} and {051}, and can form pseudohexagonal “trillings”.
Fracture: Subconchoidal / irregular.
Mohs Hardness: 6
Parting: No.
Streak: Black to dark brown.
Lustre: Submetallic, vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 5,2 – 6,65
2. Geology and Deposits
Columbite (Fe) occurs in Li- and P-rich granitic pegmatites, where columbite occurs as an accessory mineral. In these pegmatites, columbite is concentrated at the edges and tantalite in the core.
It rarely occurs in carbonatites. It is common in alluvial deposits (placers).
3. Mineral Associations
It occurs associated to minerals typical of granitic pegmatites, such as quartz, feldspars (albite (including clevelandite variety), oligoclase, microcline), micas (muscovite, biotite), beryl (including aquamarine variety), tourmaline (various varieties), fluoroapatite, garnet (spessartine), zircon and various other oxides (rutile, anatase, cassiterite, ilmenite, magnetite, hematite).
Typical for these pegmatites with Li are spodumene, lepidolite, lithiophilite, tripylite and amblygonite. In addition, it contains tapiolite, samarskite-(Y), wolframite, triplite, hisingerite, bismutite, galena, goethite, Microlite Group minerals and radioactive minerals (uraninite, torbernite, pitchblende).
4. Transmitted Light Microscopy
The data below refers to Columbite-(Fe).
Refraction indices: nα: not determined. nβ: 2.29 – 2.40 nγ: not determined.
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Reddish brown.
Relief: No information available.
Cleavage: {100} distinct and {010} less distinct.
Habits: Massive, idiomorphic crystals can form parallel or radial clusters.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: No information available.
Extinction: Paralell.
Elongation sign: No information available.
Twins: Common, contact or interpenetrating crystals. They can form clusters of 3 crystals (“trillings”), pseudo-hexagonal in shape.
Zoning: Can occur due to changes in chemical composition or the alternation of zones with different degrees of porosity.
CONVERGENT LIGHT
Character: B(-)
2V angle: Not determined.
Alterations: Columbite-(Fe) is very resistant, but in contact with uraninite, metamict edges may occur.
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: Columbite (Fe) is very hard and only slowly acquires a good polish. Its hardness upon polishing is very high, approximately the same as cassiterite, tapiolite, and quartz.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray with a brownish tint, more rarely pinkish.
Compared to the color of magnetite, the color of columbite is very similar, but the brownish tint is not as distinct.
Pleochroism: Very faint, more noticeable in intergranular contacts, which are rare.
Reflectivity: 16.7 to 17.2%.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very weak anisotropy.
Internal reflections: Sparse red internal reflections occur relatively frequently. Internal reflections are more frequent in crystals with higher Ta content; they are rarer in those with higher Nb content. Internal reflections may be yellow or brown.
May be confused with: Several other minerals. The idiomorphy of columbite facilitates its diagnosis.
Wolframite has more distinct cleavage, oblique extinction, fewer internal reflections, and even weaker pleochroism and anisotropy.
Tapiolite has the same chemical composition as columbite, but is tetragonal.
Cassiterite has many more internal reflections and frequent twinning.
Magnetite and some rare minerals are very similar.
General Characteristics:
Grain shape: Columbite is frequently euhedral, but columbites in granites are often completely anhedral.
Cleavage parallel to (100) can be seen occasionally, but only locally, especially in somewhat porous, perhaps somewhat altered, portions. Cleavage parallel to (010) is only weakly indicated.
Twinning is rare.
Extinction is straight.
Intergrowths with cassiterite are frequent.
Epitaxies (oriented intergrowths) with uraninite may occur.
Metamictic bands can be observed under intense illumination in columbites adjacent to uraninite grains.
Zoning is expected, but is visible only very rarely without chemical attack. Zoning is expressed in changes in anisotropy and in the intensity and frequency of internal reflections. Locally, without relation to zoning, there are quite porous portions where internal reflections are much more abundant. Fluid inclusions can occur in these pores.
Inclusions 1: Columbite may contain inclusions (unmixings) of uraninite, which make the mineral weakly radioactive.
Inclusions 2: Columbite may also contain inclusions of galena, hematite, ilmenite, cassiterite, rutile, tapiolite, and wolframite.
Inclusions 3: Cassiterite may contain columbite inclusions, which are actually exsolutions (unmixings). This situation is frequent in some deposits. In these situations, the cassiterite contains Ta2O5 or Nb2O5 in solid solution.
Pseudomorphs of columbite on pyrochlore and vice versa occur in deposits with pyrochlore.
Substitutions: Columbite can be replaced by tapiolite, microlite, and tantalite-(Sb).