IT is well known that large, well-crystallized sheets of muscovite mica are resistant to thermal decomposition at temperatures above 500° C. Hidnert and Dickson1, for example, reported that heating at 600° C for 1 h produced little or no change in the properties of a large number of different muscovite samples. Finely divided muscovite, however, begins to lose its structural water at an appreciable rate at temperatures as low as 400° C (ref. 2). Related dioctahedral aluminosilicates, such as the montmorillonite clays, are also dehydroxylated at temperatures below 500° C (ref. 3). This process occurs without a general destruction of the crystal lattice in dioctahedral minerals. It has been recognized4 that the rate of decomposition may be limited by diffusion of water from the lattice.