1 - Crystal chemistry and phase transitions in pyrochlore and related structures

Abstract

Many synthetic compounds with a crystal structure derived from that of fluorite have great technological importance. The structure of fluorite, AX2, is verified in minerals of the fluorite group (fluorides), the uraninite group (oxides), and four unassigned members. The names applied for minerals and synthetic materials are not always the same. For synthetic compounds with formula A2B2X6Y, the term pyrochlore is applied when the structure is cubic, and the term weberite is applied when the structure is not cubic. For the mineralogical nomenclature, the pyrochlore supergroup includes cubic and trigonal species: the pyrochlore group, where B is Nb; the microlite group, where B is Ta; the roméite group, where B is Sb5+; and the elsmoreite group, where B is W. In these groups X is dominantly O. Hydrokenoralstonite and fluornatrocoulsellite are two isolated mineral species, where B is Al and Mg, respectively, and X is F. They do not belong to the four quoted mineral groups. Almost all minerals in this supergroup are cubic with space group Fd3m. Hydroxycalciomicrolite is also cubic, but the space group is P4332. Hydrokenomicrolite occurs as cubic and trigonal polytypes, respectively, Fd3m and Rm, and hydrokenoelsmoreite as Fd3m and R. Hydroxykenoelsmoreite occurs as R and fluornatrocoulsellite as Rm. Zirconolite (three polytypes: orthorhombic Acam, trigonal P312, and monoclinic C2/c), weberite (orthorhombic Imma), and ingersonite (trigonal, P312) are not currently included in the pyrochlore supergroup, but they exhibit polytypic relationships with the pyrochlore structure. Murataite-(Y) (cubic F3m), pittongite (hexagonal P6m2), and phyllotungstite (hexagonal P63/mmc) also have structures derived from that of pyrochlore. The compounds known as delta phases have structures derived from fluorite and the formula A4B3X12. Allendeite (trigonal R) is the only mineral representative of these compounds. Fluorite, pyrochlore, and delta phases are all related. As the stoichiometry changes from AX2 to A2B2X6Y to A4B3X12, the anion-to-cation ratio decreases.