Assessing different types of disorder in carbonate minerals with vibrational spectroscopy

Xu, Ben (2015) Assessing different types of disorder in carbonate minerals with vibrational spectroscopy. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this thesis, calcite was used as a test case to study the effects of structural disorder on vibrational modes. Vibrational modes were examined using Fourier transform infrared (FTIR) and Raman spectroscopy. Structural disorder was assessed by using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measurements. Data for temperature dependent FTIR, FTIR-photoacoustic (FTIR-PAS), and XAFS measurements were collected at the Canadian Light Source Inc. (Saskatoon, SK). Four normal vibrational modes of calcite were thoroughly investigated: the symmetric in-plane stretch (v₁, Raman active), out-of plane bend (v₂, IR active), asymmetric in-plane stretch (v₃, IR active), and in-plane bend (v₄, Raman and IR active). Shapes of the v₁ and v₄ peaks are affected by long-range disorder and temperature, while the v₂ and v₃ peaks remain unchanged. By comparing the FTIR absorption spectra of different calcium carbonate polymorphs, we suggest that planar carbonate arrangements are the key to cause v₄ broadening at higher temperatures and with structural defects. To explore the effects of structural defects in more detail, crystalline domain size, lattice strain, and microstrain fluctuation values were calculated from XRD data. Long-range disorder strongly affects the vibrations in calcite. Bonding distances and coordination environments in calcite based minerals (plasters) were assessed from their XAFS spectra. Results indicate that the local environments are dependent on the secondary phases (mainly Ca(OH)₂) in them. My thesis work demonstrates that paring FTIR with other techniques is more effective for researchers. Weak peaks related to the combination and isotopic modes in a calcite FTIR absorption spectrum were also well studied. My thesis work demonstrates that weak peaks at 848 cm⁻¹ and 1398 cm⁻¹ in a FTIR absorption spectrum of calcite are due to the v₂ and v₃ vibrations of ¹³CO₃²⁻ rather than combination modes. These two peaks become stronger in spectra of calcite with a higher ¹³C concentration. Also, the ratio between areas of the 848 cm⁻¹ peak and the ¹³CO₃²⁻ v₂ peak in a FTIR transmittance spectrum match the natural abundance ratio of ¹³C:¹²C (near 1%). An interesting phenomenon is that positions of these two peaks do not maintain with ¹³C concentration changes. Theoretical calculations performed by our colleagues (Anna Hirsch and Dr. Leeor Kronik, Weizmann Institute of Science, Israel) confirm my thesis results. My thesis work shows that FTIR-photoacoustic (FTIR-PAS) spectroscopy is an appropriate technique to study weak isotopic and combination-mode peaks since they are relatively enhanced under an acoustic detector. Origins of combination-mode peaks were discussed, and quantitative analyses of the ¹³C v₂ peak were carried out based on FTIR-PAS spectra. A surprising finding is that the ¹³C v₂ mode presents crystallinity dependency, while the ¹²C v₂ mode is not.

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