Inorganic calcium carbonate precipitation: Controls on mineralogy, morphology, and trace elemental composition

Experimental work, chemical data collected on cave carbonates and associated fluids were combined with published data for inorganic calcium carbonate marine precipitates to determine the factors which control the polymorphism of calcium carbonate and the morphology and minor element incorporation in calcite.; Data from Carlsbad Caverns and experimental data of Mucci and Morse (1983) suggest that with increasing fluid Mg/Ca, higher CO{dollar}sb3sp={dollar} concentrations are needed to attain calcite precipitation. In Carlsbad Caverns, aragonite precipitation occurs over a broad range of fluid Mg/Ca ratios at lower CO{dollar}sb3sp={dollar} concentrations than does the precipitation of calcites containing the highest Mg content. In marine settings, high magnesium calcites are most abundant in high energy areas at latitudes where open ocean surface waters exhibit CO{dollar}sb3sp={dollar} maxima. In contrast, aragonite precipitation occurs at slightly greater depths and at latitudes where open ocean surface waters exhibit intermediate CO{dollar}sb3sp={dollar} concentrations. This suggests that under conditions of fluid Mg/Ca ratios greater than 1, aragonite will be precipitated. Aragonite is predominant because CO{dollar}sb3sp={dollar} concentrations are below those required to precipitate high magnesium calcite.; Changes in crystal habit are controlled dominantly by changes in degree of supersaturation. At supersaturation of less than 6, crystal habits are limited to rhombohedral forms. With increasing supersaturation, scalenohedrons characterized by extremely steep rhombohedra and hexagonal prisms are developed with elongation differences due to differences in crystal dimensions as these forms precipitate. Elongation of individual composite crystals under conditions of low supersaturation results from the migration of fluid-crystal interfaces in response to changing or oscillating water levels. Fibrous fabrics result from greater crystallite density and faster growth rates, which enhance growth interferences.; Magnesium incorporation is controlled mostly by fluid Mg/Ca, but as fluid Mg/Ca ratios increase, higher CO{dollar}sb3sp={dollar} concentrations are needed to attain calcite precipitation. These apparent relationships are in concordance with experimental observations documented by Mucci and Morse (1983). Incorporation of Sr is positively correlated with Mg content. Although Fe and Mn concentrations covary within the individual fluid and mineral phases, no relationship is observed between fluid Fe/Ca and Mn/Ca ratios and calcite mole % FeCO{dollar}sb3{dollar} and MnCO{dollar}sb3{dollar}. The lack of correlation likely represents local variations in redox conditions during the precipitation.