Occurrence of Niobium
|Niobium is almost always associated with tantalum in its natural ores, so that it will be convenient to consider the occurrence of both these elements together. There are very few niobium-bearing minerals which are free from tantalum and vice versa. |
Niobium and tantalum do not occur naturally in the free state to any extent; native tantalum containing small amounts of nickel has, however, been found in the gold washings from the Ural and Altai Mountains. The metals are found mainly as negative radicals in minerals in which the oxides of iron, manganese, calcium, and various rare earth metals, for example yttrium, thorium, lanthanum, cerium and uranium, form the bases. Titanium, zirconium and tin, as well as other rare earth metals, are also frequently present. The minerals are numerous, and are very generally distributed in small quantities over the earth - niobium in apparently greater quantity than tantalum. The largest deposits have been found in North America, Greenland, Finland, Sweden, the Ural Mountains, Bavaria, and Australia.
The most important ores are described below, and tables showing typical analyses are appended.
Analyses of Niobites
|Ta2O5||5.26||28.55||. . .||9.88|
|WO3||. . .||0.76||0.13||0.88|
|ZrO2||. . .||0.34||0.13||TiO2O. - 0.53|
|MgO||. . .||0.42||0.23||. . .|
|CaO||0.58||. . .||PbO - 0.12||. . .|
|H2O||. . .||0.16||. . .||. . .|
Niobites and Tantalites
|These are mainly ferrous salts of metaniobic acid and metatantalic acid in which the iron is more or less replaced by manganese. They can be generally formulated Fe(Mn)(NbO3)2 and Fe(Mn)(TaO3)2, i.e. the salts present are niobates and tantaIates. If the niobium is in excess they are called niobites (or columbites), and if the tantalum is in excess they are called tantalites. There is no definite line of demarcation between the two classes. Tapiolite contains only iron salts. Tin and tungsten are also frequently present in small amounts, and some samples of niobites have recently been found to contain the two higher homologues of manganese, masurium and rhenium. Niobites and tantalites are usually found in igneous rocks, and are not uncommon as constituents of cassiterite-bearing pegmatite veins. Their chief localities are the Black Hills of South Dakota, U.S.A., Greenbushes and Wodgina in Western Australia, and on the Finniss River near Port Darwin in Northern Australia, from all of which regions tantalites for industrial use have been obtained, although, in consequence of the restricted demand, the total output has been small and irregular. Single masses of niobite weighing up to 2000 lb. have been discovered in the granite veins in the Black Hills of South Dakota, while at Wodgina the tantalite occurs in crystalline masses weighing up to about 550 lb. It is collected from the surface soil of the neighbouring alluvial deposits or by quarrying the pegmatite. Most native niobites and tantalites offer considerable resistance to chemical change, and as they are both hard and tough, they occur frequently in detrital deposits. These are, however, usually overlooked unless they happen to be worked for gold or tin. |
Other places where ores are known to occur are Greenland, Bavaria, Finland, Miask in the Ural Mountains, Chanteloube near Limoges in France, California, and Colorado. For recent observations of their occurrence in the British Empire see the references cited.
Analyses of Tantalites
|Density||7.03||. . .||6.08||7.190|
|TiO2||6.20||6.53||. . .||1.38|
|MgO||. . .||Al2O3 - 1.65||. . .||. . .|
|CaO||1.32||1.47||trace||. . .|
The natural niobites and tantalites are usually black, and form isomorphous, 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. When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen. Small quantities of helium have also been found occluded in them.
|This is a crude calcium niobate which may also contain appreciable quantities of titanium, thorium and cerium, together with smaller quantities of iron, magnesium, the alkali metals, and fluorine. It does not contain chlorine, and it is of interest in that some specimens are remarkably free from tantalum. It occurs in Norway and near Miask in the Ural Mountains. The ore is brown, forms regular octahedra, is brittle, and presents a conchoidal fracture. Its density varies from 4.2 to 4.5.|
Niobates and Tantalates of the Rare Earth Metals
|These are of importance for their rare earth content rather than for their niobium and tantalum. Examples are: |
Yttrotantalite Yttrotantalite, which is found at Ytterby and other parts of Sweden, and in Norway. It is richer in tantalum than in niobium, and contains considerable proportions of yttrium, erbium, cerium, and uranium, together with calcium, iron, etc. Tungstates and stannates are also present.
Fergusonite This ore is also found at Ytterby (Sweden), and in Norway, Greenland, Texas, South Africa and Ceylon. Its composition is comparable to that of yttrotantalite, except that the niobium content is usually greater than that of the tantalum, and lanthanum is also found in the basic portion. According to Rammelsberg, fergusonite from Greenland consists of isomorphous mixtures of yttrium metaniobate, Y2O3.Nb2O5, and yttrium metatantalate, Y2O3.Ta2O5. It forms brown or black tetragonal crystals, the density of which varies from 4.3 to 5.8. It is radioactive, and glows suddenly with evolution of helium when heated from 500° to 600° C.
SamarskiteSamarskite occurs in the Ural Mountains, Mitchell County (North Carolina, U.S.A.), Canada, and India. The tantalum content is often small, sometimes nil, and the rare earth oxides, chiefly yttria and ceria, are usually present in considerable number and proportions. The ore is radioactive and contains helium. It forms black, orthorhombic crystals. The density varies from 4.2 to 6.2. It has been suggested that the niobium and tantalum are disintegration products of compounds of yttrium and cerium with the two higher homologues of manganese, masurium, and rhenium.
Euxenite, ceschynite, and polycraseEuxenite, ceschynite, and polycrase are found in Norway, and differ in composition from samarskite in that they usually contain considerable quantities of titanium. Tantalum is not always present.
WohleriteWohlerite is a niobate of calcium, iron, manganese, sodium, etc., associated with considerable quantities of zirconia and silica. It is found in Norway. Other silicates which contain niobium or tantalum are struverite and ilmenorutile.
Tin and tungsten minerals frequently contain small proportions of niobium and tantalum, which are also occasionally associated naturally with cryolite and pitchblende.