首页|Highly variable content of fiuorapatite-hosted CO32- in the Upper Cretaceous/Paleogene phosphorites (Morocco) and implications for paleodepositional conditions
Highly variable content of fiuorapatite-hosted CO32- in the Upper Cretaceous/Paleogene phosphorites (Morocco) and implications for paleodepositional conditions
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NSTL
Elsevier
Phosphorus (P) usually occurs as microcrystalline minerals of carbonate fiuorapatite (CFA) in sediments of upwelling-related areas. The Upper Cretaceous/Paleogene marine sedimentary rocks in Morocco host the world's largest economic reserves in phosphorus, which were deposited on the eastern passive margin of the central Atlantic Ocean. However, a full petrographic, mineralogical and geochemical characterization of the Moroccan phosphate minerals is yet to be determined. Here, we use optical and scanning electron microscopies, X-ray diffraction, and infrared spectroscopy, in combination with a microprobe analyzer to document carbonate (CO32-) substitutions in CFA from two distinct phosphate basins and their possible implications for paleoenvironmental conditions. The Moroccan phosphorus-rich deposits predominantly comprise porous phosphatic peloids and coated grains with a maximum size of 500 um, which in turn consist of high-density nano- to micron-sized rod-shaped and sphere-like crystallites. Bulk and in situ analyses reveal heterogeneous CO32- content in CFA at the regional- and grain-scale. Fourier transform infrared spectra indicate that CO32- accommodates only in the phosphate site within the fiuorapatite structure, as found in other natural phosphate minerals. Bulk CO(3)(2)(- )variationsin CFA between phosphate basins, ranging from 1.97 to 8.24%, are likely related to depositional conditions - and thus of supersaturation with respect to CFA - during mineral formation, transport, and redeposition. Moreover, some phosphatic coated grains have revealed alternations of CO(3)(2)(- )rich and CO(3)(2)(- )poor CFA laminae, pointing out local differences in CO32- concentrations in sediment pore waters. Interestingly, the change in shape and diminution in size of CFA crystallites caused by high content of CFA-hosted CO32- could have possibly increased the intragranular porosity, highlighting an underestimated role of CCO(3)(2)(- )on grain morphology. Considering the higher solubility of CO(3)(2)(- )rich CFA than fiuorapatite, the application of such phosphate rocks from specific Moroccan areas as rock phosphate powders may be of deep interest for long-term soil fertilization. These results further evidence the importance of studying phosphorites for economic, environmental, and scientific interests.
NSTL
Elsevier
