Apatite, Magnesian Calcite, and Silica: An Experimental Approach to their Reactivity in Natural Environments

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Apatite, magnesian calcite, and silica are, along with clays, the most abundant phases in marine sediments. Knowledge of the physical and chemical properties of magnesian calcites is necessary if we are to understand their biogenic production, the carbonate cements in the marine environment, the role of magnesian calcites in the CO2 budget of the earth, and their fate during diagenesis. Apatite stabilities and reaction rates in solution are needed because of the importance of apatite in the sedimentary cycle of phosphorus, in the formation of phosphorite deposits, and in controlling the fertility of the ocean. Stability and reaction rate data on silica will help to explain the series of modifications silica undergoes during diagenesis, and help to define the role of silica in the sedimentary cycle. Unfortunately, previous investigations in these systems have failed to produce universally accepted models of their stabilities, partly because of the inadequacy of materials used in stability and kinetic experiments. A thorough understanding of the chemistry and solid solution nature of synthetic samples is necessary before the stabilities and solution behavior of natural phases can be evaluated. Our approach is to compare the characteristics of homogeneous, well- crystallized synthetic phases to natural materials. We will extend our previous measurements on magnesian calcites to span the composition range of natural samples. We will also extend our syntheses to apatite and opal. Stability determinations will be made throughout a temperature range corresponding to diagenetic conditions. SEM, x-ray, electron microprobe, Raman, and IR analyses of both synthetic and natural samples will provide a basis for interpretation of the solution behavior of these minerals, and thus their reactions in the natural environment.