pyrochlore

Mineral oxide (Na,Ca,U)₂(Nb,Ta,Ti)₂O₆ (OH,F); sp. gr. 4.3–4.5; hardness 5.0–5.5; cubic; normally brown to black, green when heated; light brown streak; vitreous to greasy lustre; crystals usually octahedra or irregularly shaped grains; cleavage octahedral when present; found in granitic rocks and pegmatites, with or near alkaline rocks, and in association with zircon, apatite, and carbonatites. The calcium and sodium in the crystal lattice can be substituted with uranium, thorium, or the rare-earth elements.