Abstract
Phosphorus (P) usually occurs as microcrystalline minerals of carbonate fluorapatite (CFA) in sediments of upwellingrelated 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 phosphateminerals is yet to be determined. Here, we use optical and scanning electron microscopies, X-ray diffraction, andinfrared spectroscopy, in combination with a microprobe analyzer to document carbonate (CO32− ) substitutions inCFA from two distinct phosphate basins and their possible implications for paleoenvironmental conditions. The Moroccanphosphorus-rich deposits predominantly comprise porous phosphatic peloids and coated grains with a maximum size of500 μm, which in turn consist of high-density nano- to micron-sized rod-shaped and sphere-like crystallites. Bulkand in situ analyses reveal heterogeneous CO32− content in CFA at the regional- and grain-scale. Fourier transforminfrared spectra indicate that CO32− accommodates only in the phosphate site within the fluorapatite structure, as found inother natural phosphate minerals. Bulk CO32− variations in 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 CO32− -rich andCO32− -poor CFA laminae, pointing out local differences in CO32− concentrations in sediment pore waters. Interestingly, thechange in shape and diminution in size of CFA crystallites caused by high content of CFA-hosted CO32− could have possiblyincreased the intragranular porosity, highlighting an underestimated role of CO32− on grain morphology. Considering thehigher solubility of CO32− -rich CFA than fluorapatite, the application of such phosphate rocks from specific Moroccanareas as rock phosphate powders may be of deep interest for long-term soil fertilization. These results further evidence theimportance of studying phosphorites for economic, environmental, and scientific interests.