Experimental CO2 incorporation into Mg-cordierite: nonlinear behaviour of the system

Abstract

High-pressure experiments were conducted to introduce CO 2 molecules into the structural channels of a largely Al,Si-ordered Mg-cordierite. The run products were investigated by means of X-ray diffraction and infrared spectroscopy, and they were analysed for CO 2 using a coulometric method. The cell parameter c o of Mg-cordierite, which increases linearly with the CO 2 content of the mineral, is suitable to characterize CO 2 -bearing cordierites, whereas the cell parameter a o and the distortion index Δ are too sensitive to the Al,Si-ordering state of cordierite to be used for this purpose. It was found that for any investigated P-T-t condition the cordierite CO 2 content increases with the initial total CO 2 content of the capsule. Moreover, for a fixed P-T condition and a specific total CO 2 content in the capsule, the CO 2 occupancy of cordierite channels oscillates as a function of time without reaching the saturation equilibrium. These unexpected phenomena are tentatively explained by a conflict between two processes: the stabilizing one is the diffusion of CO 2 molecules into the channels, a tendency which would weaken with decreasing amount of CO 2 gas in the capsule, and the destabilizing one is under discussion. The available experimental results clearly show the nonlinearity of the system. It is however possible that the «chaotic» behaviour of cordierite described in this paper does not occur in rocks because of the more gradual changes of CO 2 contents in natural conditions