The structure of the apatite-replacing carbonate shells and oolites in experimental apatite synthesis by bacterial activity has been shown to be microspherical. Size (a fraction of one micron to several microns) and type of the microspheres (spherical or ovoid; ovoid or cauliflowerlike; isolated, siamese twinlike, or in chains ...) fit both the size and type of monocellular organisms very well. The pseudomorphic replacement of carbonates by apatite is well known in natural phosphorites; however, the detailed structure of the replacing apatite has not yet caught the eye of petrographers. Bacterial structures in phosphorites have been reported recently, but these structures were found isolated rather than accurately located within phosphatized shells. Our results show that, while the initial aragonitic structure of phosphatized shells is prismatic, the pseudomorphic apatite is microspherical, and phosphatization appears to spread beyond the shell walls towards the center of the chambers. All stages between the almost empty and the totally infilled apatite phosphatized shells could be observed in one sample from Morocco. The similarity of the apatite microstructures in experimental and natural samples lends further support to the genesis model we proposed earlier, that combination of phosphorus from RNA released by bacteria and calcium from carbonate is one of the possible genesis models of apatite in nature.

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