Crystal chemistry of rare earth elements in fluorapatite and some calc-silicates

The uptake of individual REE by fluorapatite and cuspidine has been studied by hydrothermal synthesis at approximately 700 degrees C and 0.1 GPa P (sub H2) O. The REE contents of fluorapatites crystallized with approximately 10 mol.% Ca (sub 4REE) (sub 6) (SiO (sub 4) ) (sub 6) F (sub 2) in the starting composition varies from 12 wt.% in Nd-fluorapatite to 3.2 wt.% in Lu-fluorapatite, showing preference for LREE over HREE. The uptake pattern is convex upwards, peaking at Nd, and has a double-'humped' appearance consistent with contribution from 4f crystal field stabilization energy. A similar uptake pattern is obtained for fluorapatites crystallized with approximately 2500 ppm REE in the starting composition. The experimental uptake patterns are similar to REE patterns of natural fluorapatites in some metamorphic and metasomatic rocks. Synthetic cuspidine coexisting with fluorapatite has a marked preference for HREE over LREE, and uptake is limited to the composition NaCa (sub 2REESi) (sub 2) O (sub 7) F (sub 2) . Bond valence calculation is a powerful first-approximation technique for deducing REE site preference of end-member calc-silicate mineral structures with multiple Ca positions (e.g. epidote-group minerals, vesuvianite, pumpellyite and cuspidine).