The Geochemistry of Phosphorite and Associated Rocks of Akashat Formation (Middle Paleocene) in Akashat Mine, Iraqi Western Desert. Part I: Major Oxides

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The phosphatic compounds in the phosphorites of Akashat Formation (Middle Paleocene) are mainly composed of francolite. The associated rocks (phosphatic limestone) consist of calcite and francolite. As well as, there are little amounts of dolomite, palygorskite, quartz and iron oxides. Francolite is composed of Ca, P, F and O, the tetrahedron SO4 and planar CO3 as CO3+F substitute PO4. Some of Na and Mg are substituted in both sites Ca(I) and Ca(II). F occupied its sites in the channel. Some F subsititute O in CO3 to form the pseudo-tetrahedron of CO3+F. The elements Si, Al, Mg, Fe and K represent the clay minerals; montmorillonite, palygorskite and sepiolite, as well as, quartz and iron oxides. The geochemistry of phosphorite reflects the primary neoformation conditions of marine apatite (francolite) from calcium, phosphate and fluorine, in addition to the substitution of positive mono- and di- valance cations for calcium in Ca(I) in submarine mud of the oceanic floor. The phosphate deposits were exposed to winnowing and transporting by upwelling currents to the continental shelf area. This led the phosphatic deposits to be affected by early and late chemical and biochemical diagenetic processes, causing activities in substitution of cations for calcium in Ca(II) and carbonate, also, additional fluorine and sulfates for phosphate. The flow of continental water to near-shore area, whose content is magnesium leading to remineralization of clay minerals, (e.g. palygorskite and sepiolite from montmorillonite) by depleting magnesium from sea water, as well as the activity of microorganism, all that is provided by suitable growth conditions for phosphatic compounds in continental shelf area.