CRYSTAL-CHEMISTRY, AND THERMODYNAMIC AND KINETIC-PROPERTIES OF CALCITE, DOLOMITE, APATITE, AND BIOGENIC SILICA - APPLICATIONS TO PETROLOGICPROBLEMS

Sedimentary minerals are generally metastable phases that, given time and changing environmental conditions, recrystallize to more stable ph ases. The actual pathway of stabilization is governed by a host of kin etic factors. Unfortunately, much of the theoretical and experimental work on thermodynamic and kinetic behavior of sedimentary minerals eit her has not reached field practitioners in sedimentary petrology, or h as been conducted under conditions that are difficult to extrapolate t o natural sedimentary environments. In this paper we review and presen t new data on the basic crystal chemistry, thermodynamic and kinetic p roperties of calcite, dolomite, apatite, and biogenic silica, and disc uss the relevance of these data to the solution of geological and geoc hemical problems. The crystal chemistry and structure of a given magne sian calcite exert a fundamental control on its solubility and solid s olution behavior, and this control can be seen most clearly through co mparison 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 documentatio n is the domain of much field-based study. Experimental work involving dolomite has focused on delineation of phase relations in dry and aqu eous systems at moderate to high temperatures, determination of reacti on pathways followed during dolomitization of calcium carbonate, and m easurement of reaction rate. Uncertainties reside in the relevance of these data to the classic problem of low-temperature dolomite formatio n. We suggest that the effort must now focus on designing experimental systems that effectively mimic natural environments, and yield reacti on rate data as a function of temperature and solution composition. Su ch an example is presented. A primary goal in experimental work involv ing carbonate fluorapatite has been an understanding of the mechanism of formation of this mineral. We review the state of this knowledge; a nd also present the results of ongoing dissolution rate experiments. T he importance of this work is that it bears directly on the understand ing of the role carbonate fluorapatite plays in the biogeochemical cyc le of P in the oceans. Many factors influence the solubilities of opal ine silica and the silica polymorphs, and solubility plays an importan t role in controlling silica diagenesis. A model is presented that rel ates changes in sediment properties including density and acoustic vel ocity to the stages of silica diagenesis. The model is applied to sedi ments of Deep Sea Drilling Project Leg 63. The discussions of the sedi mentary 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 react ions to form a foundation for an understanding of natural mineral-solu tion reactions; (2) experimental investigations of mineral reactions i n 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 sedimenta ry minerals in aqueous solutions and their role in reactions governing precipitation and dissolution of sedimentary phases.