Transformations of Calcium Carbonate in an Aqueous Solution

MICHIWAKI Ayako MATSUI Nobuyuki

Regardless of the method, when calcium and carbonate ions are combined, they react to form calcium carbonate. Among the three types of calcium carbonate crystals, calcite (C), aragonite (A), and vaterite (V), the order of thermodynamic stability is C>A>V. We carried out homogenous reactions of calcium chloride and urea varying the reaction times from as short as 1 hour to as long as 48 hours. We also carried out heterogeneous reactions of calcium chloride and sodium carbonate varying the reaction times from as short as 5 minutes to as long as 24 hours. We studied the products of these reactions using scanning electron microscopy (SEM) and X-ray diffraction. We were able to establish clearly that calcite, which has the highest thermodynamic stability among these three crystal forms, undergoes considerable change in its crystal structure regardless of whether it is a product of a homogeneous or a heterogeneous reaction.

In particular, we observed that regardless of the type of reaction, homogeneous (which yields calcite and aragonite crystals) or heterogeneous (which yields calcite and vaterite crystals), longer reaction times (e.g. homogeneous = 18 hours; heterogeneous = 3 hours) did not necessarily yield better ordered crystals than shorter reaction times (e.g. homogeneous = 6 hours; heterogeneous = 1 hour). Moreover, for heterogeneous reactions up to 30 minutes, X-ray diffraction showed calcite increasing and then decreasing, and vice-versa for vaterite. In contrast, for homogeneous reactions up to 2 hours, X-ray diffraction showed no calcite forming and aragonite decreasing, but for reactions between 2 and 6 hours it showed both calcite and aragonite increasing.