EXPERIMENTAL CO2 INCORPORATION INTO MG-CORDIERITE - NONLINEAR BEHAVIOR OF THE SYSTEM

High-pressure experiments were conducted to introduce CO2 molecules in to the structural channels of a largely Al,Si-ordered Mg-cordierite. T he run products were investigated by means of X-ray diffraction and in frared spectroscopy, and they were analysed for CO2 using a coulometri c method. The cell parameter c(o) of Mg-cordierite, which increases li nearly with the CO2 content of the mineral, is suitable to characteriz e CO2-bearing cordierites, whereas the cell parameter a(o) and the dis tortion index DELTA are too sensitive to the Al,Si-ordering state of c ordierite to be used for this purpose. It was found that for any inves tigated P-T-t condition the cordierite CO2 content increases with the initial total CO2 content of the capsule. Moreover, for a fixed P-T co ndition and a specific total CO2 content in the capsule, the CO2 occup ancy of cordierite channels oscillates as a function of time withoUt T eaching the saturation equilibrium. These unexpected phenomena are ten tatively explained by a conflict between two processes : the stabilizi ng one is the diffusion of CO2 molecules into the channels, a tendency which would weaken with decreasing amount of CO2 gas in the capsule, and the destabilizing one is under discussion. The available experimen tal results clearly show the nonlinearity of the system. It is however possible that the ''chaotic'' behaviour of cordierite described in th is paper does not occur in rocks because of the more gradual, changes Of CO2 contents in natural conditions.