Abstract
Essential for the production of phosphate fertilizers, sedimentary phosphate rocks (i.e. phosphorites) are vital to the world's growing food needs. These rocks can contain significant amounts of rare earth elements (REE), considered critical metals and used as geochemical markers. However, in phosphorites, their distribution, speciation and enrichment processes remain unconstrained and dissensual. The aim of this thesis is to better understand these issues in Moroccan phosphorite deposits.This work focuses on 30 phosphorite samples from the Gantour and Tarfaya basins in Morocco, and is based on a multi-analytical approach combining: petrology, mineralogy and geochemistry, using methods such as optical microscopy, SEM, XRD, ICP-MS, ICP-OES, EDS and LIBS; and X-ray spectroscopy using synchrotron methods, such as SXRF, XANES, and EXAFS. Yttrium will be selected as a proxy for HREEs, and lanthanum as a proxy for LREEs.REEs are mainly found in fluorapatite and are distributed very heterogeneously among and within the grains, with local concentrations ranging from around ten to several hundred ppm. The most enriched grains/zones are associated with mesoporous, nanocrystalline fluorapatite, enriched in CO32- groups replacing PO43-. These enrichments can represent almost half of the total REE balance.REEs substitute for crystallographic calcium sites in the fluorapatite crystal lattice, causing significant distortion. LREEs preferentially substitute the Ca(1) site, while HREEs preferentially substitute the Ca(2) site. In their direct atomic environments, REEs are found systematically associating with B-type carbonate defects present in fluorapatite crystals (i.e. CO32- replacing PO43-). Heterovalent REE3+-Ca2+ substitution is balanced by the formation of Ca and O vacancies and coupled substitutions that may involve Na+ or F-.Regarding enrichment processes, the nano-crystallinity and mesoporosity of fluorapatite grains favor a greater reactive surface and the development of a hydrated layer promoting fluid-solid exchanges. Epitaxial growth of apatite from this hydrated layer quantitatively incorporates the CO32-, REE3+, REE(CO3)2- and REE(CO3)+ ions available in seawater/interstitial water, enabling their charge equilibria in the solid. Phosphatogenesis at the SWI (i.e. sediment-water interface) coincides with slow crystallization kinetics of nanocrystalline carbonate-fluorapatite, and maximum dissolved REE contents in the fresh sediment column. A process of grain winnowing may also occur, resulting in fluorapatite grains being maintained in the zone near the SWI, where the enrichment conditions are met.The results obtained in this thesis constitute a breakthrough in our understanding of the crystallochemistry of REEs and their enrichment in phosphorites, and particularly in carbonated fluorapatite. Similar studies could be further conducted on other apatitic materials such as teeth, fossilized bones or deep ocean sediments; or on other trace elements sensitive to redox/pH conditions, such as uranium or cerium.