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Calcium carbonate is known to be one of the sedimentary materials occuring with phosphorite in many of the phosphorite deposits. This relationship has been described for various geological periods and at various scales: mineable phosphorite alternating with barren carbonate; calcific or more often dolomitic phosphorite alternating with phosphatic dolomite or limestone. In the present study, we attempt to explain why there is a link between phosphorite and carbonate. This relationship does not presuppose a genetic link between minerals, although this link does exist where apatite can be recognized as the result of apatitization of pre-existing calcite. Calcite is a true biomineral that is directly produced by living marine organisms as their skeletons. It is deposited as a solid material before various diagenetic recrystallizations add their imprints. Apatite is an authigenic mineral, precipitated during early diagenesis from an organic matter (OM)-rich ooze. The OM must be furnished predominantly by naked and or siliceous organisms and its degradation processes are controlled by dysoxic conditions. The formation of limestones depends on a predominantly carbonate-producing productivity that is common because of the abundance of carbon. Phosphorite formation requires high quantities of OM related to specific noncarbonate-producing productivity that is less common bccause the availability of phosphorus is minor compared to that of carbon. The “phosphorite factory,” which produced the phosphorite deposits, and the “carbonate factory” are independent but complementary rather than competing processes; they do not use the same primary material, nor do they occur at the same phase of the sedimentary cycle. The occurrence of alternations of beds of phosphorite and beds of calcium carbonate is thought to be the record in the sediment of changes in the dominant type of bioproductivity in the water column. Shallow-water shelves without significant detrital influx are the environments where such frequent changes in the type of bioproductivity are known to occur easily in response to slight physical or chemical changes.

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