Temperature Dependence Of IR Absorption Coefficient And Incorporation Mechanism Of OH In Nominally Anhydrous Minerals: In Situ FTIR Experiments At Varying Temperatures

The knowledge of OH incorporated in nominally anhydrous minerals (NAMs) is crucial for understanding the chemical and physical properties of the Earth's interior. The H in NAMs is mobile and the speciation, physicochemical properties and sites in the crystal structures may vary with temperature. So it is indispensable to analyze H in NAMs at different temperatures. IR is a powerful tool to detect trace amount of OH in NAMs because of its high sensitivity, so, it is widely used to measure water content in NAMs and explore H incorporation mechanism. Thereby, in situ varying temperature FTIR technique is a very good method to detect H in NAMs at different temperatures. Although some researchers have investigated temperature dependence of OH and H2O absorption in some minerals and glasses, the study concerning possible behavior of OH absorption in natural NAMs at varying temperatures is still scarce. In this dissertation, we choose such common minerals as clinopyroxene, orthopyroxene, feldspar and rutile to identify H speciation using Micro-FTIR method at room temperature. Then chemical compositions are analyzed by use of EMPA for minerals with high water content. At last, in situ FTIR experiment is carried out to investigate the variation at varying temperatures (from RT to 500℃at 100℃increments) using heating/cooling stage attached to IR microscope. Moreover, polarized IR measurement is carried out for typical samples. In order to investigate the temperature dependence of IR absorption coefficient and incorporation mechanism of OH in NAMs, the followings are completed step by step:(1) The behavior of structural OH in clinopyroxene (cpx) and orthopyroxene (opx) during successive heating, cooling and stepped heating has been investigated by in situ Micro-FTIR measurements under temperatures ranging from 25℃to 500℃at 100℃increment. The results suggest that both Cpx and Opx exhibit a decrease in OH stretching vibration frequency with increasing temperature, and the change is reversible. Shift in absorption band frequency is controlled by thermal expansivity of the OH site or lattice interstice.(2) Based on the FTIR experiment results at varying temperatures for diopsides and omphacites, we discussed the incorporation mechanisms of OH defects in clinopyroxene mineral from a new point of view ---- thermal expansion. There are three groups of OH absorption bands in clinopyroxene: (1) 3600-3620 cm^-1; (2) 3500-3540 cm^-1; and (3) 3445-3465 cm^-1. The OH incorporation mode of group 1 band is Si⁴⁺+O²-+1/2 H₂-Al³⁺+OH^-, while the M2 vacancy is responsible for the OH incorporation mode of group 3 band. The OH incorporation mode of group 2 band is complex and probably relates to several different positions. The OH dipole vibration direction of group 2 band is the same with group 3 bands, along shared edge of M1 and M2 polyhedra O2-O1. And the OH dipole of group 1 band vibrates between O2 and O3 along edge of M2 polyhedron.(3) The behavior of structural OH in clinopyroxene (augite and omphacite) during successive heating has been investigated by in situ Micro-FTIR measurements under temperatures ranging from room temperature to 500℃at 100℃increment. The first group of OH band (3620-3640 cm^-1) exhibits a systematic decrease of peak position upon successive heating, while the other two groups (3520-3535 cm^-1, 3450-3465 cm^-1) show only little change. Both augite and omphacite display a decrease of integral absorbance of OH fundamental stretching vibration upon successive heating. The IR spectra of OH band are reversible when the temperature decreases from 500℃to room temperature, suggesting that changes in IR indicate changes in molecular state of OH and no loss of OH happens. The change of integral absorbance of OH bands indicates that OH absorption coefficient is temperature dependent, so it is necessary to apply different absorption coefficients when determining OH content from Beer-Lambert law at different temperatures and sample temperatures should be reported in quantitative IR studies.(4) Temperature dependence of IR absorption of OH in nominally anhydrous orthopyroxene and feldspar has been investigated by in situ Micro-FTIR measurements under varying temperatures ranging from 25℃to 500℃at 100℃increment. The results demonstrate that the shift direction of OH peak position is related with initial wavenumber. Integral absorbances of OH in orthopyroxene and feldspar decrease with increasing temperature. Although the trend is similar to that of clinopyroxene, the magnitude of temperature response of OH integral absorbances of these minerals is variable. The magnitude of temperature responses are correlated with wavenumbers: the lower wavenumber bands have stronger temperature dependence of integral absorbances. The changes of IR spectra of OH band are reversible, so the change of integral absorbance with temperature indicates temperature-dependent IR absorption coefficient of OH in nominally anhydrous minerals (NAMs). (5) In view of the limitations of previous works and in order to deeply investigate the IR behavior of NAMs at different temperatures, we choose a very simple mineral-rutile, not only carry out in situ high temperature FTIR measurements, but also in situ low temperature FTIR, polarized FTIR and high temperature XRD. The combined results suggest that the H site of rutile is on the (001) plane. At room temperature, the H site of 3297 and 3279 cm^-1 band is on (1/2, 0, 0) and (1/2, 1/2, 0) respectively. In addition, the H bond of 3279 cm^-1 band is nonlinear. The change of areas for these bands shows opposite dependency on temperature, indicating two possibilities: (1) H site will transfer at varying temperatures and (1/2, 1/2, 0) site is stable at high temperature, while the other site (1/2, 0, 0) is stable at low temperature; or (2) the IR absorption coefficients of two bands display opposite temperature dependency.