Roger D. Aines, George R. Rossman
Division of Geological and Planetary Sciences, 170-25,
California Institute of Technology,
Pasadena, California 91125, U.S.A.
We have studied the behavior of molecular water and carbon dioxide in the channels of cordierite and beryl at temperatures up to 900°C using high temperature infrared spectroscopy. Above 400°C water that is structurally bound in the channels begins partitioning into an unbound state with the characteristics of a gas. The process is fully reersible and involves both type I and type II water in both minerals. Dehydration occurs after most of the water ins in theis unbound state, and channel cations are no longer coordinated by the type II water molecules. These cations can then move ito the wall of the channel or be expelled from the channel, opening the channel for dehydration of the water contained in it. This behavior is contrasted with that of mucsovite, in which the hydroxide shows no change in speciation and only slight changes in its spectroscopic properties at temperatures below the dehydration point. CO2 in the channels of cordierite does not undergo major changes in bonding at high temperatures. Although all the water in the cordierite was released, about 40% of theCO 2 remained aftger heating to 800°C. Heating to 900°C was requried to expel all CO2. This is indicative of the tighter wedging of CO2 in the channels. Because of an equilibrium among type I, type II, and unbonded gas-like water at high temperature, the concentration of type I alone serves as an indicator of the water fugacity. The type II concentration only responds to the number of channel cations and need not be considered in water fugacity calculations. Cordierites with greater numbers of channel cations will effectively close to re-equilibration at higher temperatures, making them more suitable as indicators of water and carbon dioxide fugacity.