Calcium carbonate phase analysis using XRD and FT-Raman spectroscopy

There is a need for the quantitative determination of the ternary mixtures of calcium carbonate polymorphs (calcite, aragonite and vaterite), which are present in a number of materials including limestones, industrial scale formation and several pathological cases. Application of infrared spectroscopy proved to be inadequate, due to band overlapping, while use of Raman spectroscopy and X-ray powder diffraction (XRD) has been reported only for the calcite-aragonite mixture. In order to address the problem, a comprehensive non-destructive methodology for the simultaneous quantitative determination of the calcium carbonate crystal phases in their ternary mixtures based on the use of Fourier transform Raman spectroscopy (FT-RS) was developed. Binary mixtures were used to construct the calibration graphs using the Raman bands at 711 cm(-1) for calcite, 700 cm(-1) for aragonite and 750 cm(-1) for vaterite. Detection limits were found to be 0.13, 0.18 and 1.3 mol.-% for calcite, aragonite and vaterite, respectively, while the relative errors in determining the mol.-% of a known ternary mixture of calcium carbonate polymorphs were 1.6% for calcite, 0.3% for aragonite and 1.1% for vaterite. A similar methodology was developed using XRD. XRD calibration graphs were constructed using the 104 reflection of calcite, the 221 reflection of aragonite and the 110 reflection of vaterite. Detection limits for calcite, aragonite and vaterite were found to be 0.90, 2.90 and 6.90 mol.-%, respectively, while the relative errors in determining the mol.-% of the same ternary mixture used for testing the FT-RS methodology were 2.5, 1.8 and 0.6%, for calcite, aragonite and vaterite, respectively.