The spectroscopic analysis of vaterite and other forms of calcium carbonate

Hollett, Mark Joseph (2000) The spectroscopic analysis of vaterite and other forms of calcium carbonate. Masters thesis, Memorial University of Newfoundland.

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Abstract

Precipitation of calcium carbonate from supersaturated solutions of CaCl₂ with K₂CO₃, Na₂CO₃ and (NH4)₂(CO₃) was studied in detail. In the course of our studies on the natural and synthetic vaterite forms of calcium carbonate, it has been possible to prepare, isolate and characterize the calcite, vaterite and aragonite forms of CaCO₃, as well as the hydrates, CaCO₃⋅6H₂O and CaCO₃⋅1H₂O. The precipitation reactions were found to be unexpectedly complex, with a number of different possible products. This is in contrast to the fact that calcite is the only thermodynamically stable solid in equilibrium with the saturated solution under ambient conditions. -- The existence of each form was confirmed by various analytical and spectroscopic techniques. The crystalline forms, calcite, vaterite and aragonite, were characterized by such methods as differential scanning calorimetry (DSC), micro-Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), thermogravimetry with evolved gas analysis (TG-EGA) and powder X-ray diffraction (XRD), whereas the hydrated forms, CaCO₃⋅6H₂O and CaCO₃⋅1H₂O, were only characterized by DSC and Raman. It was also found that the various forms could be distinguished visually by the appearance of the solutions upon mixing and by the crystal habits observed under the microscope. -- Optimum precipitation conditions were determined for the formation of each polycrystalline form The tendency for the metastable phases to precipitate from various aqueous, supersaturated solutions was dependent upon factors such as the temperature, concentration of reactants, duration of precipitation, as well as the stirring rate. However, it has been determined through the course of several repeated experiments that these kinetically-controlled precipitations were mainly dependent upon the temperature and the initial concentrations of the reactants. -- Raman and infrared spectroscopic studies of natural and synthetic vaterite indicate an extremely complicated crystal structure in which the carbonate ions appear to occupy a number of different crystallographic sites. The spectroscopic results are consistent with the structure reported by Meyer (1969). However, we suggest that the disordered stacking sequence is not random over the 12 carbonates of the unit cell, but rather follows a pattern characteristic to that of an incommensurate phase. A detailed unit cell group analysis was not possible because the vibrational spectrum is further complicated by intermolecuiar coupling of carbonate ions on different lattice sites. Raman and infrared spectra for the regions of the internal modes of the carbonate ions indicate that the different sites fell into three major groups with differing occupancies. For example, in the v₁ region, three peaks were observed at 1075.0, 1081.4 and 1090.9 cm⁻¹ with relative intensities 0.40:0.19:1.00. These multiple site effects were confirmed by studies of ¹⁸O and ¹³C carbonate impurities. It would appear that the vaterite structure is similar to the incommensurate phase of γ-Na₂CO₃.Trace amounts of water were detected in the freshly prepared vaterite, but are not believed to be necessary for kinetic stability. Laser fluorescence measurements were also employed to estimate the concentration of manganese(II) impurities in the precipitates. -- This research has permitted clarification of previous studies and has led to a more in-depth understanding of the mechanism of metastable crystal growth.

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