Infrared investigation of CO2-bearing cordierites

CO₂ molecules were introduced experimentally into the structural channels of synthetic well ordered Mgcordierite (Mg₂ ^VI](Al₄Si₅)^IV]O₁₈]) at temperatures of 600 and 800° C, and pressures of 7, 8, 10, 12, and 25 kbar. Powder infrared spectra of the run products show five absorption bands in the region of the asymmetric stretching mode of CO₂. Two of them, strong and sharp, occurring at 2353 cm^–1 {2} and 2348 cm^–1 {3}, are related to two different types of CO₂ molecules. The relative intensity of the band {2} (type I) increases with the cell parameter c_o whereas the relative intensity of the band {3} (type II) increases with the parameter a₀ of the crystal. It is concluded that CO₂ molecules of type I may be oriented with their elongation parallel to the c-axis of the crystal, while CO₂ molecules of type II lie with their O-C-O vector parallel to the a-axis. Analytical data indicate that the intensity ratio Z of these two bands ({2}/{3}) is a linear function of the CO₂ content of cordierite. This ratio depends also on the temperature and, to a less extent, on the pressure under which cordierite entrapped CO₂ molecules. It is proposed to combine this infrared parameter Z together with an estimate of the P-T conditions of the incorporation of CO₂ into the channels, in order to determine the CO₂ content of natural cordierites. The samples do not need to be of high purity and only small amounts (<5 mg) are necessary. This semi-empirical analytical method, which does not require complicated data treatments, is suitable for CO₂-rich cordierites of granulite facies rocks.