THE CRYSTAL-STRUCTURE OF CHERALITE-(CE), (LREE, CA, TH, U) (P,SI)O-4,A MONAZITE-GROUP MINERAL

Cheralite-(Ce), the title compound, is a monazite isostructure with st oichiometric Th4+; the phase is important because it is the natural an alog of a proposed solid-state repository for radioactive actinides. T he atomic arrangements of two samples of cheralite-(Ce), including mat erial from the type locality, have been refined to R values of 0.022 a nd 0.024. The phase crystallizes in space group P2(1)/n, and no orderi ng of substituents causing a decrease in symmetry is noted. Like monaz ite, the atomic arrangement is formed of distorted, 9-coordinated REE (Th) polyhedra linked to P (Si) tetrahedra in [001] chains. Charge-bal ance for the REE(3+)<----> Th4+ substitution is maintained by concomit ant substitutions of Ca2+ in the REE site and Si4+ for P5+ in the tetr ahedral site. With the substitution of smaller constituents (Th-[9](4) = 1.09 Angstrom and Ca-[9](2+) = 1.18 Angstrom for Ce-[9](3+) = 1.19 6 Angstrom) in the REE site, contraction of that site occurs principal ly along interchain equatorial bonds in (001). Intrachain bonds to the phosphate tetrahedra along [001] shorten to a lesser extent because t he increased cation repulsion of the Th4+-P5+-Th4+ sequence in the che ralite-(Ce) chains, as opposed to the REE(3+)-P5+-REE(3+) sequence in monazite, hinders contraction of those bonds.