THE INFLUENCE OF CARBONATE BRINES ON CA-MG EXCHANGE AND SUBSEQUENT CACO3 PRECIPITATION

This project tests the influence of dissolved carbonate on the exchange of Ca²⁺ for Mg²⁺ from interlayer sites of montmorillonitic clays. Such exchange is hypothesized to be the first step in one possible mechanism of authigenic clay formation (Esquevin, 1958; Millot 1970). This has implications for understanding both ancient and modern mineral precipitation and diagenesis in alkaline, evaporative lacustrine settings, such as those of the Kenyan Natron-Magadi Basin. A series of experiments is being conducted to test this idea. Clay Mineral Society standard montmorillonite (SWy-2) was first Ca-saturated by suspension in calcium chloride solution, and then subjected to an array of varying brines of increasing Mg²⁺ and HCO₃^-/CO₃^2- concentration, thus allowing for subsequent Ca²⁺ for Mg²⁺ exchange at interlayer sites, and precipitation of CaCO_3(s) upon Ca²⁺ release. Experiments are underway, and XRD, ICP-AES, and XRF will be used to analyze fluid, clay, and carbonate products; MgCO₃ brine solutions were subject to ICP-AES, and pH analyses. Two trials at lower dissolved Mg²⁺ were performed initially at: 0 mg/L, 103.8 mg/L, 207.6 mg/L, and 311.3 mg/L for comparative purposes. From this several changes in the XRD patterns of the clays were observed, including: leftward shift of 001 peaks and decrease in intensity of the 001 with brine concentration, increased peak heights relative to Swy-2 standard; potentially resulting from interlayer expansion, and widening from distinct peaks to humps of 032 peaks as determined by X’Pert HighScore. Samples will be glycolated for confirmation of results, serving as a baseline for the succeeding brines of higher concentration run at Mg²⁺ concentrations of: 0 mg/L, 360 mg/L, 720 mg/L, and 1080 mg/L. Temperatures of ~40°^C were used in later trials in an attempt to increase dissolved Mg²⁺concentration in brines, thus increasing the potential for cation exchange within clays. In ongoing experiments, the significance of the proportion of Mg:Ca in solution on crystallography of the CaCO3 structure (Berner, 1975; Mucci and Morse 1983), such as formation of High-Magnesium Calcite (HMC), Low-Magnesium Calcite (LMC), and aragonite will also be examined. The interlayers of the clays from the low Mg²⁺ trials are seen to expand in the 2:1 structure implying successful Ca²⁺ for Mg²⁺ exchange.