Abstract

Conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid My/Ca ratios and crystal morphology, crystal composition, and carbonate mineralogy, resulting in the formation of meteoric, equant, low-magnesium calcite and marine, acicular, high-magnesium calcite and aragonite. Problematically, calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals, and aragonite may coexist with calcite in either environment. Commonly, however, a systematic relation exists between crystal morphology, composition, mineralogy, and rates of reactant supply to growing crystal surfaces. For example, equant rather than acicular crystals of calcite form in modern, deep and/or cold marine, meteoric-phreatic, and deep-burial settings where the degree of carbonate saturation and/or rates of fluid flow are low. In areas of higher saturation and/or fluid flow, such as in warm, shallow-marine and meteoric-vadose environments, acicular calcite may predominate. This relation is also seen in systems in which aragonite and calcite form in intimate association. Aragonite precipitation is favored when rates of reactant supply are high; calcite forms when rates are low. Such relations suggest that crystal morphology, composition, and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants, principally carbonate ions, at growth sites. Precipitating phases are the ones which can best accommodate such excess reactants; ambient Mg/Ca ratios only indirectly control the nature of inorganically precipitated carbonate phases.

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