any of a group of minerals that are natural compounds in which Ta and Nb form the compound. Tantaloniobates have the generalized formula AnBmXp, where A represents the large cations (Ca2+, TR3+ [rare-earth elements], U4+, Th4+, Na +, or, less frequently, Pb2+, Sb3+, or Bi3+) or medium cations (Fe2+, Mn2+, or Mg2+), B represents Nb5+ and Ta5+, which may be replaced by Ti4+, Sn4+, or Fe3-, and X stands for O2-, OH-, or F-. With a close degree of covalence for the bond between the anions X and the cations of groups A and B, tantaloniobates approach the multiple oxides.
The class of tantaloniobates comprises more than 100 mineral species and their varieties. The main minerals are columbite, tantalite, loparite, and pyrochlore; ixiolite, (Nb,Ta,Sn,W,Sc)3O6; samarskite; euxinite, (Y,TR)(Nb,Ta,Ti)2O6; and microlite, Ca2(Ta,Nb)2O6(F,OH). The structural basis of the tantaloniobates is niobium-oxygen or tantalum-oxygen octahedrons, (Nb,Ta)O6, bonded together by cations of group A.
Tantaloniobates containing U, Th, and TR are metamict and strongly radioactive and contain a variable quantity of water. Polymorphous modifications are common. Tantaloniobates form fine inclusions in rocks. Large segregations are rare; crystals are typical of loparite, pyrochlore, and columbite-tantalum. The coloration of tantaloniobates is black, dark brown, or brownish yellow; simpsonite is light yellow, and manganotantalite is pink. The minerals are usually semitransparent or translucent, with a vitreous luster; the metamict varieties have a resinous luster. The hardness of tantaloniobates on Mohs’ scale ranges from 4.5 to 8 but is usually 5–6; that of the metamict varieties is lower. Densities are 3,500–9,000 kg/m3, depending on the Ta and Ti content. Tantaloniobates that contain Fe, Mn, and rare-earth elements are weakly magnetized.
Tantaloniobates are accessory minerals in granites and nepheline syenites; they form higher concentrations in granitic pegmatites, alkaline pegmatites, rare-metal albitized granites, feldspar metasomatites, carbonatites, lujauvrites, and foyaites. They are the ores for Ta, Ni, and rare-earth elements.
REFERENCEGeokhimiia, mineralogiia i genetkheskie tipy mestorozhdenii redkikh elementov, vol. 2. Moscow, 1964. Section 11.
A. I. GINZBURG