The magnesian calcite in surface sediments from the central Tyrrhenian Sea is almost entirely authigenic. Its concentration decreases with depth, irregularly and rapidly in the shallower samples but very slowly in the deeper ones. The X-ray diffraction peaks of the magnesian calcite become progressively irregular and polymodal with depth, as index of precipitation of more poorly crystallized phases. On the basis of considerations about the rates of sedimentation and the mineralogical characters of the authigenic phases, a decrease of the rate of precipitation of the magnesian calcite with depth has been hypothesized.

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