a group of minerals, complex oxides containing columbium (niobium) and tantalum. The chemical composition varies; the approximate formula is (Ca, Na, U, Ce, Y)2–m(Nb, Ta, Ti)2O6(OF),–m.nH2O. U, Th, Ce, Y, Fe, and other elements are present in isomorphous admixture form and as adsorbed oxides. Numerous pyrochlore varieties are differentiated according to composition, for example, uranpyrochlor, betafite, mendelyeevite (rich in U, Ti), microlite (rich in Ta), obruchevite, and koppite (rich in rare-earth elements). Pyrochlores usually occur in the form of octahedral crystals of the isometric system and as grains or aggregates. They range in color from light yellow, reddish-brown, and dark brown to almost black. The crystal structure resembles that of fluorite. The hardness and density vary according to composition and physical state. The hardness ranges from 3 to 5.5 on Mohs’ scale, and the density from 3,700 to 5,000 kg/m3; microlite has a density as high as 6,400 kg/m3. Pyrochlores are sometimes highly radioactive and often occur in altered form, in which case they are classed as metamict minerals.
Pyrochlores occur in syenite or nepheline-syenite pegmatites in association with zircon, ilmenite, sphene, calcite, and other minerals. When found in large aggregates, they may serve as the ore in the extraction of columbium and tantalum.