Many carbonate particles from the tropical Brazilian continental shelf contain intragranular magnesian calcite cement, with 13 mole percent MgCO 3 (d (104) lattice spacing of 2.99 A) in solid solution in the calcite. In aragonitic Halimeda plates, precipitation occurs within the utricles; upon complete infilling, as much as 50 percent of the plate is composed of secondary magnesian calcite. Filling of these Halimeda plates by magnesian calcite can play an important role in the preservation of the plates: mechanical removal of the primary aragonitic skeleton exposes the cemented utricles, resulting in thinner knobby plates with greater resistance to subsequent abrasion. The wide distribution of intragranular cementation on the Brazilian shelf suggests that cementation by magnesian calcite is potentially a significant pathway for the flux of carbonate from seawater into sediments.

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