Cation Diffusion in Fluorapatite

Diffusion of manganese and uranium has been characterized in natural and synthetic fluorapatite under dry conditions. The source of diffusant for Mn experiments were mixtures of ground synthetic or natural fluorapatite and MnO powder, heated in sealed silica glass capsules prior to diffusion anneals. Mn experiments were run by sealing source and apatite in silica glass capsules under vacuum, and annealing capsules in 1 atm furnaces for times ranging from thirty minutes to a few months, at temperatures from 650 to 1050°C. The Mn distributions in the apatite were profiled by Rutherford Backscattering Spectrometry (RBS). The following Arrhenius relation is obtained for Mn diffusion in natural Durango fluorapatite, for diffusion parallel to c: D_Mn = 5.4x10^-7exp(-288 kJ mol^-1/RT)m²sec^-1. Mn diffusion normal to c appears to be similar to diffusion parallel to c, and diffusivities in natural and synthetic fluorapatite are the same within experimental uncertainties. Uranium diffusion experiments were run with a U-doped microcrystalline apatite source, made through solid-state reaction of UO₂, CaF₂ and Ca₃(PO₄)₂ under buffered (NNO) conditions. The source and apatite specimens were loaded into Pt capsules, then sealed under vacuum in silica glass capsules with a solid buffer (NNO). Preliminary results over the temperature range 1150-1250°C yield the following Arrhenius relation: D_U = 1.4x10^-2exp(-511 kJ mol^-1/RT)m²sec^-1. Diffusivities of Mn are comparable to those of Sr (Cherniak and Ryerson, 1993), and slightly slower than Pb (Cherniak et al., 1991) in apatite. The ionic radii for divalent Mn, Sr, and Pb are 0.90, 1.21 and 1.23 Å, respectively, in 7-fold coordination (Shannon, 1976), and 1.00, 1.31, and 1.35 Å in 9-fold. The similar diffusion rates for Sr and Mn, despite their significant differences in cationic radii, suggest that cation size does not exert strong influence on diffusion of divalent cations in apatite, a finding consistent with that observed for the trivalent REE. Cation charge, however, does seem to more significantly influence diffusivities in apatite. U diffusion is about 4 orders of magnitude slower than Mn diffusion, and about 2 orders of magnitude slower than REE diffusion (Cherniak, 2000). Further, these results suggest that the activation energy for U diffusion is significantly higher than those for divalent cations or trivalent REE. Cherniak et al. (1991) GCA 55, 1663-1673; Cherniak and Ryerson (1993) GCA 57, 4653-4662; Cherniak (2000) GCA 64, 3871-3885; Shannon (1976) Acta Cryst. A32, 751-767.