Calcium carbonate polymorphism: New insights into the role of solution saturation state and composition (magnesium/calcium) on calcium carbonate mineralogy, morphology and fabrics

Abstract

For many decades researchers have devoted major efforts in an attempt to better understand the factors controlling the precipitation of differing calcium carbonate mineralogy, morphology, and fabrics, as well as the incorporation of Mg2+ into calcite precipitating in natural systems. In the study presented here, a novel experimental approach was used to delineate the role of instantaneous supersaturation and solution Mg/Ca ratio in calcium carbonate mineralogy and composition. The results of this study show that in solutions with different Mg/Ca ratios, the transition between the aragonite and the calcite + aragonite precipitation fields is controlled by a combination of the supersaturation state of the solution with respect to calcite and the Mg/Ca ratio in solution. The data suggest that in order for calcite to precipitate from a solution with high Mg/Ca ratio (such as modern seawater), a significantly higher degree of supersaturation is required than that of a solution with lower Mg/Ca ratio (e.g. fresh water). The transition in mineralogy is attributed to a relative decrease in calcite growth rates as a result of increasing incorporation of Mg2+ at higher solution Mg/Ca ratios.

In addition, the relationship between saturation state (Ω) solution composition and the formation of different crystal morphology and fabrics was quantified. Calcite morphology is controlled by the interplay between solution supersaturation and Mg/Ca ratio, while aragonite crystal morphology seems to be unaffected. The development of different calcite and aragonite fabrics can be attributed to changes in solution supersaturation, calcite growth rates and fluid flow regimes.

The results of this work were used to evaluate the presumed morphological and textural differences between calcium carbonate crystals produced by biogenic activity and those growing through abiotic processes. Crystal morphology and fabrics are not effective tools as the sole means to determine the biogenic origin of calcium carbonate. These findings are relevant to present-day mineralogy distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogy changes through geologic time. The observed dependence of calcium carbonate mineralogy in solution composition and supersaturation suggests that carbonate mineralogy is controlled by kinetics of growth.