SOME COMMENTS ON THE CHEMISTRY AND STABILITY OF INTERSTRATIFIED ILLITE-SMECTITE AND THE ROLE OF OSTWALD-TYPE PROCESSES

Much emphasis has been placed recently on the role of Ostwald-type pro cesses (both ripening and the step rule) in the formation of interstra tified illite-smectite clay minerals during diagenesis. Closer investi gation of the relationships between illite particle thickness (as defi ned in terms of the fundamental particle theory) and particle chemistr y show that simple Ostwald ripening is too simplistic a mechanism to d escribe observed changes in illite thickness and composition. In addit ion, available thermodynamic data on clay minerals suggest that the co nventional notion of metastability associated with Ostwald's step rule is an inaccurate description of the relationship between illitic clay minerals and their macroscopic 2:1 phyllosilicate analogues.