Infrared study of CO₂ incorporation into pyrophyllite [Al₂Si₄O₁₀(OH)₂] during dehydroxylation

We report infrared spectroscopic observations of the incorporation of CO₂ into pyrophyllite that has been beated between 200°C and 1250°C for periods of 15 min, 1 h and 5 days. The presence of CO₂ is characterized by the V₃ band of CO₂ near 2347 cm^-1, detectable in samples in which dehydroxylation has commenced after heating above 450°C. With increasing temperature, the CO₂ signal becomes more intense. The signal reaches its maximum intensity near 800°C with an annealing time of 15 min. Further heating leads to a decrease in the CO₂ signal and the occurrence of an extra signal near 2156 cm^-1 that implies the presence of CO. The process is characterized by significant time-dependence. indicating its kinetic nature. The peak positions of CO₂ signals show systematic variations with temperature. Our results suggests that the CO₂ molecule is associated with the local structure rather than being present as free gaseous CO₂, and that the local structure of pyrophyllite is gradually modified during high-temperature treatments. However, no signals related to carbonate molecules (CO²-₃) were detected. The results suggest that CO₂ or other carbon-based molecules may diffuse into some clay minerals during dehydroxylation and may become altered due to structural modifications at high temperatures. This may have significance for possible CO₂ sequestration in shales and clay formations.