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INFRARED STUDY OF CO₂ INCORPORATION INTO PYROPHYLLITE [Al₂Si₄O₁₀(OH)₂] DURING DEHYDROXYLATION

¹ College of Materials and Bioengineering, Chengdu University of Technology, Chengdu 610059, Sichuan, PR China
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
³ Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia

^* E-mail address of corresponding author: mz10001{at}esc.cam.ac.uk

We report infrared spectroscopic observations of the incorporation of CO₂ into pyrophyllite that has been heated between 200°C and 1250°C for periods of 15 min, 1 h and 5 days. The presence of CO₂ is characterized by the ₃ 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 suggest 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 (CO₃^2–) 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.

Key Words: CO₂ • CO • Dehydroxylation • Infrared Spectroscopy • Pyrophyllite • Sequestration

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