INSTANTANEOUS SUPERSATURATION EXPERIMENTS: INVESTIGATING THE ROLE OF CO₃^2- CONTROLLED SUPERSATURATION ON HIGH MAGNESIUM CALCITE AND ARAGONITE PRECIPITATION

An instantaneous supersaturation experimental setup designed to replicate the precipitation of carbonate minerals in natural systems was used to investigate the role of supersaturation on the precipitation of high magnesium calcite (HMC) and aragonite from solutions with Mg/Ca ratios of ~ 1. Instantaneous supersaturation was induced by CO₂ degassing as solutions dripped into an ambient with a lower pCO₂. Solution composition and temperature were kept constant and continuously monitored alongside with pH and alkalinity.

In two experimental runs it appears that a HMC with ~ 8 mol % MgCO₃ and aragonite co-precipitated. HMC precipitated as equant rhombohedral crystals, while aragonite precipitated in bundles of needles in which individual crystals have either an acicular pyramidal or a tabular prism crystal habit. However, the precipitated assemblages differ in two key aspects: the relative abundance of HMC vs. aragonite, and their crystal size. HMC dominated the mineralogy in experiment ER1 in which most of the crystals were twins with more than double the size of the crystals in experiment ER2.

Since parameters such as temperature and Mg/Ca ratio were kept constant, the observed differences in crystal size and mineralogy distribution between experiments can only be correlated to differences in solution supersaturation at the time of precipitation. The degree of supersaturation in experiment ER1 (W=5.25) was greater than in experiment ER2 (W=3.72). Both the size and relative abundance of the twined HMC crystals in experiment ER1 suggest fast growing conditions, which can be related to the higher solution supersaturation. So at high solution supersaturations HMC growth rates are faster than those of aragonite and the sample mineralogy is dominated by HMC. Conversely, aragonite’s growth rate dominates that of HMC at lower supersaturations, and the sample mineralogy is dominated by aragonite. Based on these findings, we propose a conceptual model that uses changes in solution supersaturation to explain the precipitation of CaCO₃ polymorphs from solutions at low Mg/Ca ratios and constant temperature. Despite the fact that the solution supersaturation and calcite growth rate were significantly greater in experiment ER1, there were no changes in the MgCO₃ content of the precipitates.