Diffusion of Th was characterized in synthetic monazite under dry conditions. The

synthetic monazites (either pure CePO4, NdPO4, or a mixed LREE phosphate

containing Ce, Nd, and Sm) were grown via a Na2CO3–MoO3 flux method. The

sources of diffusant for the experiments were either synthesized ThSiO4 or

CaTh(PO4)2 powders mixed with CePO4. Experiments were performed by placing

source and monazite in Pt capsules and annealing capsules in 1atm furnaces for

times ranging from 3 weeks to a few hours, at 1350 to 1550C. The Th distributions in the monazite were profiled by Rutherford back-scattering spectrometry. The

following Arrhenius relation was obtained for Th diffusion in monazite:

D(m2/s) = 67 exp[-740(kJ/mol)/RT]

The diffusivity of Th was found to be similar for monazites containing a single

REE (Ce or Nd) and the mixed LREE phosphates. Th diffusion was also similar for

experiments run using the Th silicate and Ca–Th phosphate sources, suggesting

that the substitutional mechanism for Th in monazite, i.e., Th+4+Si+4→REE+3+P+5

for the ThSiO4 source, and Th+4+Ca+2→2REE+3 for the CaTh(PO4)2 source, did not

significantly affect Th diffusivities, and that Th was likely the rate-limiting species

in diffusion. The Th diffusion in monazite was about 3 orders of magnitude slower

than Pb diffusion. This contrasted with the findings of Gardés et al. (2006), who

determined that diffusivities measured in (Pb, Th)↔REE exchange (which would

under most circumstances be rate-limited by the slowest-diffusing species) were

comparable to those measured by Cherniak et al. (2004) for Pb. The Th diffusion in

zircon was about an order of magnitude slower than in monazite, but with a similar

activation energy for diffusion. The lower diffusivities in zircon may be a

consequence of the larger disparity in size between Th and the Zr site in zircon as

compared with Th and the REE site in monazite. Nonetheless, Th was essentially

immobile in monazite with respect to exchange by volume diffusion under most

geologic conditions. These findings may also have implications for containment of

high-level actinide-based nuclear waste in monazite ceramic waste forms, but it

should be noted that mechanisms other than diffusive exchange (e.g.,

recrystallization, dissolution/re-precipitation) may be important to consider along

with diffusion when evaluating the retentivity of monazite for Th and other actinide

elements.

Th Diffusion in Monazite. D.J.Cherniak, J.M.Pyle: Chemical Geology, 2008,

256[1-2], 52-61