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Fabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X‐ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrated.

Detailed petrographic study of the Upper Cretaceous phosphatic chalks of northern France, particularly by scanning electron microscopy, shows that phosphate occurs predominantly as phosphatized bacterial remains. Shiny phosphatic crusts on hardgrounds and anisotropic phosphate coatings on grains are interpretated as phosphate mineralized microbial (‘microstromatolitic’) structures. Phosphatic remains of ovoid bacteria and of microbial colonies with botryoidal surfaces have been observed in chambers of foraminiferans and bryozoans, and within other bioclasts, and occur as pore linings in peloids, coprolites and lenses of phosphatic chalk incorporated into hardgrounds. The microbial community which formed the crusts and grain coatings developed by surface accretion, whereas the other community grew solely in the intragranular porosity of grains and in the extragranular porosity of semilithified sediments. Analysis of these two types of microbial growth, which have also been documented from other phosphorite deposits, permits a better but as yet incomplete understanding of the mechanisms of phosphatization.

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