FORMATION PROCESS OF CACO₃ - CAMG(CO₃)₂ CARBONATE MINERAL FROM ARAGONITE AND SOLUTION

Dolomite, CaMg(CO₃)₂, commonly occurs in replacing calcite by dolomitization. The dolomitization is controlled by many parameters including temperature, pressure, dissolution rate of precursor materials and ionic strength of the solution. Nucleation and growth rate of dolomite in dolomitization are apparently subjects to parameters on the solid - solution reactions. In this study, we conducted aqueous reaction experiments with natural aragonite crystals and CaCl₂-MgCl₂ solutions. Powdered aragonite crystals are sealed in teflon or hastelloy bombs with 1M, Ca:Mg=1:1 solution at 100, 150 and 200 degrees C. Run durations are up to 16 days. Run products are observed with powder XRD and SEM/EDS analysis. Less than 150 degrees C, aragonite crystals does not show any reaction with the solution. At 200 degrees C, aragonite is lost within 24 hours. Low magnesian calcite (LMC) and high magnesian calcite (HMC) appear by XRD with extinction of aragonite. The LMC is a metastable product that occurs within 12 hours only. From 24 hours of run duration, diffraction peak of dolomite grows with the decreasing of that of HMC. In the SEM/EDS observation, dissolution textures of aragonite and HMC are observed. The HMC is also an intermediate product before precipitation of dolomite from the solution. The Mg / (Mg + Ca) ratios of HMC and dolomite are approximately 0.30 and 0.45, respectively. The Mg content of crystals increases from Mg-free aragonite, the starting material, to dolomite through the intermediate metastable crystal phases. Precipitation processes of LMC, HMC and dolomite should be controlled by local exchange of cations with the precursor crystals and solutions.