Formation of ${\text {CaCO}}_3$ varieties from a carbonated aqueous solution

Abstract

Experimental studies have been conducted to clarify the paragenetic conditions of ${\text {CaCO}}_3$ varieties, calcite and aragonite formed from the same wall rock. In addition, the solubility dependence of calcite, aragonite, and limestone formed from limestone cave in ${\text {CO}}_2$-saturated solutions was compared with the previous studies. First, the calcite and aragonite were powdered to below 200 meshes. In addition, the powders were, respectively, added into the ${\text {CO}}_2$-saturated distilled water at 8, 13, 18, and $22 \,^\circ {\text {C}}$ and were kept constant for 15 days. The amount of dissolved ${\text {CaCO}}_3$ was then measured. The solubility of aragonite was higher than that of calcite, and the solubility decreased exponentially with increasing temperature. Simultaneous production and growth of calcite and aragonite proceeded only when the saturation of aragonite was reached in aqueous carbonate solution. Conversely, when the dissolved amount of ${\text {CaCO}}_3$ has not been reached, calcite is only formed and grown. In these solutions, aragonite crystals dissolve and calcite crystals grow. The degree of saturation of aragonite can be reached by rapid evaporation of solvent, ${\text {CO}}_2$ gas degassing, and the presence of Mg. However, paragenesis of calcite and aragonite in natural caves can be explained by rapid evaporation of the solvent. A rapid evaporation of the solvent may occur at a place, where a small amount of karst solution flows and a large amount of air flows. In such places, paragenesis of calcite and aragonite is possible.