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Solid Solution Behaviour of Synthetic Monazite and Xenotime from Structure Refinement of Powder Data.

ABSTRACT In order to study the complex substitution behaviour observed in natural samples, synthetic structural analogues of the rare earth element (REE) orthophosphate minerals monazite and xenotime have been prepared. Compounds of the type: (Nd,Yi_,)POb , (La,Yi.,)PO4 , (Sm,Yi_,)POd and (Nd,Ybi_,)PO4 have been prepared both by precipitation and solid state methods and then heated at temperatures of 1000, 1200 and 1500 “C. Structure refinement of powder X-ray data, using Rietveld full profile methods, has been carried out on these mixed light- and heavy-REE phosphates to obtain cell parameters and relative proportions of co-existing phases. The lever law was used to estimate the extent of heavy-REE substitution in synthetic light-REE-containing monazite phases and light-REE substitution in heavy- REE-containing xenotime phases. In general, the larger the difference in radii of the two REEs in each phase, the lower the extent of REE substitution, A higher level of both heavy-REE substitution in monazite phases and light-REE substitution in xenotime phases was found in preparations heated to 1500 OC. Xenotime phases, containing Y in combination with smaller light-REEs, which have been heated at 15OO”C, show some evidence of REE ordering, but this behaviour has not yet been well characterised. INTRODUCTION Monazite, a Light Rare Earth Element (LREE) phosphate mineral, crystallises in the monazite structure type with a monoclinic unit cell and a space group P2l/n (no. 14) (Ni et al., 1995). The REE atom is in nine-fold coordination with the oxygen atoms of slightly distorted phosphate tetrahedra. The Heavy REE (HREE) phosphate mineral, xenotime, crystallises in the more dense zircon structure type with a tetragonal unit cell and a space group 14l/amd (no. 141) (Kristanovic, 1965). The REE atom is in eight-fold coordination with the oxygen atoms of phosphate tetrahedra. The effective ionic radius (r) of the REE atom determines which of the two structure types are formed. The larger radii LREEs crystallise together with phosphate in the monazite structure, whilst the smaller radii HREEs crystallise in the zircon structure (Miyawaki and Nakai, 1993). We have prepared synthetic structural analogues of these minerals containing different combinations of light and heavy REEs to determine the composition range of the structural boundary, and, decide if any REE ordering occurs within the phases. The crystal chemical limits of HREE substitution in monazite and of LREE in xenotime have been determined by use of powder X-ray diffraction techniques. The Rietveld method was used to obtain the unit cell parameters and relative proportion of each phase within a particular compound.