Crystal chemistry of Rare Earth Elements in fluorapatite and some calc-silicates
Fleet, Michael E.; Pan, Yuanming
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 591 - 606
published: May 19, 1995
manuscript accepted: Jan 20, 1995
manuscript received: Aug 8, 1994
ArtNo. ESP147050703013, Price: 29.00 €
Abstract The uptake of individual rare earth elements (REE) by fluorapatite (FAp) and cuspidine has been studied by hydrothermal synthesis at about 700°C and 0.1 GPa P(H2O). The REE content of fluorapatites crystallised with approximately 10 mol% Ca4REE6(SiO4)6F2 in the starting composition varies from 12 wt% in Nd-FAp to 3.2 wt% in Lu-FAp, showing preference for LREE over HREE. The uptake pattern is convex upward, peaking at Nd, and has a double-"humped" appearance consistent with contribution from 4/crystal field stabilisation energy. A similar uptake pattern is obtained for fluorapatites crystallised 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 NaCa2REESi2O7F2. 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. The REE characteristics of fluorapatite and some important metamorphic calc-silicate minerals (epidote-group minerals, vesuvianite, pumpellyite and cuspidine) are consistent with predictions from bond valence and size of Ca positions.