Niobite-tantalite

The element is found in niobite (or columbite), niobite-tantalite, parochlore, and euxenite. Large deposits of niobium have been found associated with carbonatites (carbon-silicate rocks), as a constituent of parochlore. Extensive ore reserves are found in Canada, Brazil, Nigeria, Zaire, and in Russia. 

Complex oxides Chromite niobite-tantalite pyrochlore ilmenite wolframite scheelite... 

Niobium is relatively rare in the Earth s crust, with tm abundance of roughly 20 mg/kg (i.e., ppm wt.), such as in cobalt and Hthium. The element, owing to its strong chemical reactivity with oxygen, never occurs free in nature and is found in a combined form in niobite (i.e., columbite) [(Fe,Mn)(Ta,Nb)j03, orthorhombic], tapiolite [Fe(Ta,Nb)203, tetragonal], niobite-tantalite, pyrochlore [(Na,Ca)2(Nb,Ta,Ti)j0 (0H,F).H20], and euxenite [(YJir,Ce,U)(Ta,Nb)TiO, orthorhombic]. Large deposits of niobium have been found associated with carbonatites (i.e., carbonates-silicates rocks) as a constituent of pyrochlore. 

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

Niobium (formerly known as columbium) and tantalum occur together in the mineral columbite (Fe,Mn)(Nb,Ta)20g when Nb-rich, it is called niobite and when Ta-rich, tantalite. Fusion of the ore with alkali gives poly-niobates and -tanta-lates, and further treatment with dilute acid yields Nb205 and Ta205. One method of separation utilizes the more basic character of Ta at a controlled concentration of HF and KF in aqueous solution, the oxides are converted to... 

Most important mineral Columbite is a continuous mineral series with niobium and tantalum. Niobite (Fe,Mn)(Nb,Ta)jOj with 78% Nb Oj is end mineral" on the Nb-side. (Figure M32). Compare tantalite in the tantalum chapter. 

Niobium and tantalum are present in the earth s crust as oxides in minerals that form a continuous series between the two extreme minerals niobite (Fe,Mn) (Nb,Ta)20g with 78% NbjOj and tantalite (Fe,Mn)(Ta,Nb)20g with 84% TajOj. A common name for all minerals within the series is columbite (Figure M32). 

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