CRYSTAL-CHEMISTRY OF THE MONAZITE AND XENOTIME STRUCTURES

Monazite and xenotime, the RE(PO4) dimorphs, are the most ubiquitous r are earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-pre cision structure studies of both the natural phases and the synthetic RE(PO4) phases for all individual stable rare earth elements. Monazite is monoclinic, P2(1)/n, and xenotime is isostructural with zircon (sp ace group I4(1)/amd). Both atomic arrangements are based on [001] chai ns of intervening phosphate tetrahedra and RE polyhedra, with a REO(8) polyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a REO(9) polyhedron in monazite that pre ferentially incorporates the larger light rare earth elements (La-Gd). As the structure ''transforms'' from xenotime to monazite, the crysta llographic properties are comparable along the [001] chains, with stru ctural adjustments to the different sizes of RE atoms occurring princi pally in (001). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [0 01]. In that projection, the chains exist in (100) planes, with two pl anes per unit cell. In monazite the planes are offset by 2.2 Angstrom along [010], relative to those in xenotime, in order to accommodate th e larger light RE atoms. The shift of the planes in monazite allows th e RE atom in that phase to bond to an additional O2' atom to complete the REO(9) polyhedron.