Crystal chemistry, and thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica: applications to petrologic problems

Abstract

Sedimentary minerals are generally metastable phases that, given time and changing environmental conditions, recrystallize to more stable phases. The actual pathway of stabilization is governed by a host of kinetic factors. Unfortunately, much of the theoretical and experimental work on thermodynamic and kinetic behavior of sedimentary minerals either has not reached field practitioners in sedimentary petrology, or has been conducted under conditions that are difficult to extrapolate to natural sedimentary environments. In this paper we review and present new data on the basic crystal chemistry, thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica, and discuss the relevance of these data to the solution of geological and geochemical problems. The crystal chemistry and structure of a given magnesian calcite exert a fundamental control on its solubility and solid solution behavior, and this control can be seen most clearly through comparison of synthetic and biogenic phases. Thus, variations in crystal chemistry and structure, through solubility control during diagenesis, yield a range of possible stabilization pathways, whose documentation is the domain of much field-based study. Experimental work involving dolomite has focused on delineation of phase relations in dry and aqueous systems at moderate to high temperatures, determination of reaction pathways followed during dolomitization of calcium carbonate, and measurement of reaction rate. Uncertainties reside in the relevance of these data to the classic problem of low-temperature dolomite formation. We suggest that the effort must now focus on designing experimental systems that effectively mimic natural environments, and yield reaction rate data as a function of temperature and solution composition. Such an example is presented. A primary goal in experimental work involving carbonate fluorapatite has been an understanding of the mechanism of formation of this mineral. We review the state of this knowledge, and also present the results of ongoing dissolution rate experiments. The importance of this work is that it bears directly on the understanding of the role carbonate fluorapatite plays in the biogeochemical cycle of P in the oceans. Many factors influence the solubilities of opaline silica and the silica polymorphs, and solubility plays an important role in controlling silica diagenesis. A model is presented that relates changes in sediment properties including density and acoustic velocity to the stages of silica diagenesis. The model is applied to sediments of Deep Sea Drilling Project Leg 63. The discussions of the sedimentary phases calcite, dolomite, apatite, and biogenic silica in this paper point to several directions for future experimental research on sedimentary mineral-solution reactions. These include emphasis on: (1) experimental studies of synthetic sedimentary mineral-solution reactions to form a foundation for an understanding of natural mineral-solution reactions; (2) experimental investigations of mineral reactions in aqueous solutions under conditions of composition, temperature, and pressure similar to the natural conditions of mineral formation; and (3) studies of the surface (as opposed to bulk) properties of sedimentary minerals in aqueous solutions and their role in reactions governing precipitation and dissolution of sedimentary phases.