Determination of the Composition of Calcite-Vaterite Mixtures by Infrared Spectrophotometry

Quantitative analysis of mixtures of the crystalline polymorphs of calcium carbonate—vaterite, aragonite, and calcite—is of general interest. For example, in connection with kinetic studies of binary systems of these crystalline modifications there is need for rather fast and accurate methods for the determination of the composition of mixtures of the polymorphous forms mentioned above. Rao and Murthy some years ago published a paper where they describe a method for quantitative analysis of calcite-aragonite mixtures making use of infrared spectrophotometry and the alkali halide pellet technique. In connection with kinetic studies of the transformation of vaterite to calcite, we have developed a rather fast and precise procedure for quantitative analysis of vaterite-calcite mixtures. Like the method of Rao and Murthy, it is based on infrared spectrophotometry, employing the baseline method for obtaining the absorbances of the bands, but differs in some details from their procedure. After the conclusion of the present work, Xyla and Koutsoukos published a paper dealing with quantitative analysis by means of infrared spectroscopy of mixtures of calcium carbonate polymorphs including vaterite-calcite mixtures. As in the present work, the KBr pellet technique has been employed, but band areas have been used as a measure of the content of the polymorphs in the mixtures. For calcite-vaterite mixtures the areas under the bands at 746 cm−1 and 877 cm−1 were taken as a measure of the content of vaterite and calcite, respectively, although the latter band coincides with the vaterite band at 877 cm−1. I t is not obvious what justifies this procedure, nor is it explained how the band areas were obtained and overlapping bands separated.