Crystal chemistry of the monazite and xenotime structures

Abstract Monazite and xenotime, the RE(PO4) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(PO4) phases for all individual stable rare earth elements. Monazite is monoclinic, P21/n, and xenotime is isostructural with zircon (space group I41/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REO8 polyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a REO9, polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the structure “transforms” from xenotime to monazite, the crystallographic properties are comparable along the [001] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (001). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 Å along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional O2′ atom to complete the REO9 polyhedron.