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PRINCIPAL PUBLICATION AND AUTHORS
Sorption model for yttrium in fluorapatite: Geochemical implications, C. Bonnet (a,b), M. Muñoz (a), O. Mathon (b), V. Motto-Ros (c), A. Elghali (d), F. Parat (a), J. Aubineau (a), J.-L. Bodinier (a,d), Geochem. Persp. Lett. 27, 1-7 (2023); https:/doi.org/10.7185/geochemlet.2326 (a) Geosciences Montpellier, Université de Montpellier, CNRS, Montpellier (France) (b) ESRF (c) ILM, Institut Lumière Matière, Université Claude Bernard Lyon 1, Lyon (France) (d) GSMI, Geology and Sustainable Mining Institute, Mohammed VI Polytechnic University, Ben Guerir (Morocco)
REFERENCES
[1] J.M. Hughes et al., Am. Mineral. 76, 1165-1173 (1991). [2] Y. Pan et al., Rev. Mineral. Geochem. 48, 13-49 (2002). [3] B. Reynard et al., Chem. Geol. 155, 233-241 (1999). [4] M. Muñoz et al., Am. Mineral. 88, 694-700 (2003). [5] J. Blundy et al., Earth Planet. Sci. Lett. 210, 383-397 (2003).
Regarding the hydrothermal-magmatic settings, the results from H-FAp suggest that partitioning models involving the Ca(1) site for HREE are inappropriate to explain the typical HREE-depleted normalised pattern typically observed [1]. Instead, it is proposed that the Ca(2) site should control HREE incorporation and fractionation. This can affect subsequent theoretical calculations of partition coefficients of REE between FAp and hydrothermal fluids, and the resulting geochemical implications (e.g., fluid compositions, crystallising temperature, etc.) [5].
Regarding S-FAp, the Y speciation models involving either the mixed Y(2) + Y-ads or the Y(2)-carbonate model, can both explain the typical unfractionated normalised pattern (the so-called past seawater pattern ), characterised by an HREE enrichment relative to LREE [3]. However, the Y(2)-carbonate model (Figure 107b) is particularly relevant as REE are mainly complexed by carbonate groups in seawater. A quantitative incorporation of REE-carbonate groups, along with vacancies and lattice distortion, may indeed favour the fluid-conservative behaviour observed for REE in S-FAp.
Further in-situ X-ray absorption spectroscopic studies of the REE speciation in sedimentary FAp are required to validate the REE-carbonate model. In addition, this will be valuable for examining REE behaviour in temporally laminated structures containing FAp, such as phosphorite grains, polymetallic nodules or other biogenic apatites, commonly used to unravel past seawater conditions and their temporal changes like pH, Eh, or ocean circulation.
This study demonstrates that X-ray absorption spectroscopy can provide new insights for the development of an advanced geochemistry.
Fig. 107: Fluorapatite crystal structure projected along the c-axis showing yttrium speciation models. a) H-FAp: Y(2); (b) S-FAp: Y(2)+Y-adsorbed model; (c) S-FAp: Y(2)-carbonate model.
