Synthetic and naturally occurring calcium and magnesium carbonate minerals are widely used in a range of industrial and environmental applications where the mineral quality and purity is often critical for their intended use. Hence accurate characterisation of the mineral assemblages is essential. Equally important is an understanding of the chemical and physical pathways leading to mineral formation and their roles in carbon sequestration from greenhouse gases. This study investigates the application of Raman and infrared spectroscopies to Ca-Mg carbonate analysis.
A full quantitative calibration has been achieved for quaternary mixtures by Raman spectroscopy (RS) employing monovariable and multivariable methods. The method was validated by X-ray powder diffraction (XRD). The lowest error on component values was obtained by Principal Component Regression with application of Standard Normal Variate. The quantifications show that RS is comparable to XRD.
The effect of particle size on the fundamental vibrations of the [CO32-] anion in calcite is investigated by mid-infrared and RS. While the effect of particle size on the infrared signature of internal modes of the [CO32-] anion is well documented, this thesis documents associated changes in Raman spectra as a function of particle size. With decreasing size spectral contrast diminishes and changes in the relative ratios of the internal modes occur. For RS the turnaround from optically thick to thin material occurs in the 42-59 μm size range with further changes occurring at ≤ 5 μm.
RS was also utilized to monitor carbonate reaction kinetics after dissolution of [Mg(OH)2] by CO2 sparging in the presence of calcium salts at 35 °C, 30 days duration. Four experiments employing different calcium salts, Ca:Mg ratios and effect of hydromagnesite [Mg5(CO3)4(OH)2.xH2O] seeding were examined utilizing vibrational spectroscopies, XRD and SEM. Results suggest that carbonate mineral paragenesis is driven by geochemical feedback between a range of calcium and magnesium carbonate dissolution-precipitation events where decomposition of nesquehonite [Mg(HCO3,OH)∙2H2O] leads to formation of magnesium carbonate hydrates [Mg5(CO3)4(OH)2.xH2O]. XRD confirmed that these hydrated phases contain 8 and/or 5 molecules of crystalline water. However, RS cannot distinguish these phases. Traces of barringtonite [Mg(CO3)∙2H2O] found at the end of experiments were interpreted as an indicator of incongruent dissolution of nesquehonite. Findings suggest that the Raman active ν1 mode of barringtonite is situated at ca. 1094-1095 cm-1.
The limitations of Raman analysis in the context of mineral assemblage quantification, short range ordering and particle size effects are discussed in the context of these findings.
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