Résumé
Fluorapatite (FAp), which occurs in various geological settings, contains rare earth elements (REE) for which unveiling the crystal chemistry is a key geochemical issue, especially for unravelling the conditions of fractionation and crystallisation. However, no consensus has been reached regarding their binding modes in FAp, with studies suggesting REE 3þ-Ca 2þ substitution at the Ca(1) site, or at the Ca(2) site, or involving adsorption mechanisms. Our contribution here is based on yttrium K-edge X-ray absorption spectroscopy (XAS), performed on two genetically contrasting minerals: a hydrothermal FAp (from Durango, Mexico), and a sedimentary phosphorite (from Morocco). The results clearly show that Y substitutes for the Ca(2) site in both FAp. However, the spectral differences observed for the sedimentary FAp (B-type carbonated-FAp) suggest a sorption model that is either i) a mixture of Y-Ca(2) substitution and Y-adsorbed as an inner shell complex, or ii) Y-Ca(2) substitution along with carbonate groups replacing phosphate groups in the surrounding atomic shell. These models of yttrium sorption in FAp shed new light on the understanding of rare earth partitioning and enrichment processes, with major geochemical implications such as i) the identification of crystallising fluids and temperature in magmatic-hydrothermal settings, and ii) preservation of past seawater-porewater conditions in sedimentary settings.