(also Mendeleevite), a mineral of complex composition, consisting mainly of the niobotitanate of Ca, U, and the rare-earth elements. Named in honor of D. I. Mendeleev, it belongs to the cubic series of the pyrochlor group. Its general formula is mA2-mB2O6F1-n nH2O, where A = Ca, U, TR and B = Nb, Ti, Ta. The mineral contains up to 26 percent U3O8
Mendelyeevite forms brown and black irregular grains and masses, although octahedral and rhombododecahedral crystals are also found. The mineral is radioactive. As a result of metamict disintegration, the material in the crystals resembles a solid colloid and contains a variable quantity of sorbed water, H2O. Mendelyeevite recrystallizes on heating to 800°C, after which X-ray photographs show the cubic structure of the crystal lattice. The mineral has a hardness of 4.5-5 on Mohs’ scale and a density of 3,800-4,800 kg/m3. A rare mineral, it is found in some types of granitic pegmatites in association with zircon, euxenite, and other rare-earth minerals.