pyrochlore

Pyrochlore is a mineral group of oxide minerals defined by the general formula A2B2O7. In the ideal pyrochlore structure, A is a large trivalent cation, commonly a rare-earth element such as Nd, Gd, or Y, or an actinide such as U or Th. B is a smaller cation with valence +5, typically Nb5+ or Ta5+, though Ti4+, Zr4+, and Hf4+ occur in some members. The crystal structure is cubic, space group Fd-3m, derived from a fluorite-type lattice with ordered occupancy of the A and B sites and two distinct oxygen positions. The A-site cation is eightfold coordinated, while B sits in BO6 octahedra, together forming a robust framework that gives rise to the characteristic O7 stoichiometry.

Pyrochlore occurs in a variety of igneous and alkaline rocks, especially carbonatites, nepheline syenites, and peralkaline granitoids, and can also occur as an accessory phase in metamorphic rocks. It forms well-developed crystals that are typically brown to yellow and exhibit a vitreous to resinous luster. Pyrochlore often coexists with fluorite, zircon, apatite, and magnetite and can be a significant Nb–Ta ore in certain deposits. The mineral group shows extensive solid-solution on both A and B sites, yielding a wide range of end-members such as Nd2Nb2O7, Gd2Ti2O7, Y2Ti2O7, and Bi2Ti2O7.

In geology and materials science, pyrochlore and related solid solutions are studied for their durability and ability to incorporate actinides, making them candidates for nuclear waste immobilization. They are also of interest in solid-state chemistry and condensed-matter physics due to their magnetic and dielectric properties.