INFRARED-SPECTRA OF AMORPHOUS AND CRYSTALLINE CALCIUM-CARBONATE

The infrared spectrum of an amorphous, rather unstable form of calcium carbonate has been obtained in the region 4000-400 cm-1 and at the temperature of liquid nitrogen. In addition the IR spectra of the crystalline polymorphs (vaterite, aragonite and calcite) have been measured on powders with higher resolution and precision than previously, in the region 4000-200 cm-1 without cooling and in the region 4000-400 cm-1, cooling the samples by liquid nitrogen. The lack of observed pronounced doublet structure of the nu-3 band for CO3(2-) in vaterite and aragonite is explained as being due to the overlapping of the two component bands (nu-3a and nu-3b) by combination bands. A frequency value for the infrared-inactive normal vibration nu-1 of CO3(2-) in calcite has been obtained from the observed frequencies of two combination bands. More lattice frequencies than previously reported have been obtained for calcite and aragonite in the region 400-200 cm-1. A revised assignment for calcite is given. For vaterite, lattice bands have been measured for the first time and an assignment of the bands to lattice modes is proposed on the basis of a factor group analysis of the external modes of CO3(2-).