Notes Composition of the Controlling Phase in Muscovite Equilibrium Solubility

The equil ibrium ratio of 3.93 for pSi(OH4~ p K +] was significantly different from the ratio of 3.40 predicted by the equation for the dissolution o f muscovite of this particular composit ion in the presence of gibbsite. A new dissolution equation was derived in which the muscovite composit ion was adjusted to satisfy the solute activity data, the presumption being that muscovite of a new composit ion had formed during the lengthy equilibration period and that this new muscovite controlled the equilibrium. Although a change in muscovite composi t ion from Ko.9AI2.9Sia ~O~0(OH)2 to Ko.~Al2.gSi3.20~o(OH)2 was inferred, it could not be verified directly by X-ray powder diffraction, infrared spectroscopy, or electron microscopy. Incongruent solubility and the substitution of Si 4+ for A13+ and K + are of considerable interest because they imply that muscovite can be degraded to an illitelike composit ion under surface conditions. The present study tests the conclusions of Mattigod and Kittr ick (1979) concerning the incongruent dissolution of muscovite and the inferred change in muscovite composition by statistical studies of their muscovite analyses and solubility data and by chemical analysis o f the products of their experiments. Mattigod and Kit tr ick 's solutions were initially undersaturated but were never far from equil ibrium dur-