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The peloidal texture common to submarine substrates lithified by magnesium calcite appears to be the result of the same processes responsible for the deposition of the cement. These peloids do not, as reported earlier, represent the deposition of internal sediment of fecal or unknown origin. Rather, they are the physicochemical product of the precipitation of calcite from seawater, as indicated by: (1) the widespread occurrence of peloidal calcites in a variety of marine environments; (2) the generally limited size range of the peloids; (3) the well-developed zonation of peloidal textures in many cement crusts; (4) the presence of peloids in restricted microcavities; and (5) the chemical similarity of peloids and associated magnesium calcite dentate rim cements.
Although these magnesium calcite peloids resemble aragonite peloids formed by rapid repeated nucleation in experimental precipitation of aragonite from supersaturated seawater, their rates of formation must differ because pore waters are incapable of spontaneously providing the calcium carbonate required for the extensive deposition of magnesium calcite found in restricted submarine settings. Observation of magnesium calcite precipitating on experimental substrates placed on the ceiling of a submarine cave suggest that clotting or nucleation of magnesium calcites may be a very slow process, perhaps commonly involving the nucleation within an initial submicrocrystalline calcite "dust" precipitate. The final state consists of the precipitation of dentate microcrystalline rim cement arou d the peloid centers.
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