A number of feldspars and other alumino-silicate minerals were exposed to CO2 charged percolating water under high pressures and temperatures in a reaction chamber. A study of the end products indicated that the least stable of these materials readily broke down to form clay minerals, the greatest reaction rate occurring at 300°C. and a maximum CO2 pressure. It was interesting to notice that the parent minerals were stable at temperatures outside of the range of 250°C. to 350°C. The production of clay minerals similar to, if not identical with, kaolinite, beidellite, sericite, pyrophyllite and gibbsite was accomplished. The resulting end product seems to be determined by the nature of the parent rock as well as by the temperature and acidity during the reaction.