Evolution of OH groups in diopside and feldspars with temperature

Abstract

Using in situ FTIR and heating/cooling stage, we have investigated the temperature evolutions of OH bands in natural diopside, plagioclase and anorthoclase. The frequencies of three main resolved OH bands (3645, 3464 and 3361 cm −1 called band 1, 2 and 3, respectively) in diopside shift linearly with temperature. With increasing temperature, band 1 shifts to lower wavenumbers, band 2 exhibits almost no shift while band 3 shifts to higher wavenumbers. The integral absorbance of band 1, 2 and 3 decreases by 18.8, 10.1 and 31.4 %, respectively, from −50 to 400°C. No simultaneous growth and decline of OH bands is observed during the whole process, excluding H proton transfer. In contrast to OH defects in diopside, the bands at lower frequencies in feldspars weaken while their bands at higher frequencies strengthen with increasing temperature, indicating H protons change positions in the crystal structure from one site with stronger H bond to another site with weaker H bond. These results reflect different local environments of OH defects and temperature dependent OH absorption coefficients in diopside, and change of H sites in feldspar structure with temperature. It provides important constraints on the microscopic mechanisms of hydrogen incorporation in these minerals.