Smectite to illite conversion rates: effects of solution chemistry

Abstract--Mixed-layer illite/smectite (I/S) was formed by reacting the Chambers or Polkville montmoril-lonite hydrothermally at270 ~ and 350 ~ from several hours to more than 15 weeks. Reactions were con-ducted in closed vessels containing K or mixed K-Na, K-Ca, or K-Mg solutions of varying concentrations. The reaction rate and the rate of ordering of I/S for the reaction smectite + K § ~ mixed-layer I/S + SiO2 was inhibited by the addition of Na § Ca 2+, and Mg2+; the inhibitory strength of Na +, Ca 2 § and Mg 2+, on an equivalent basis, increased approximately in the ratio 1:10:30. The first order reaction-rate constants for the reactions at 270 ~ and 350 ~ indicate an activation energy of about 30 kcal/mole. In the experimental system studied, the reaction smectite- ~ mixed-layer I/S appeared to proceed by solid state transformation, as suggested by: (1) rapid dissolution of large amounts of silica, probably creating an AI-enriched residue; (2) similarity of particle size and morphology of the mixed-layer products to those of the original montmorillonite, implying no extensive dissolution of A13+; and (3) relatively high activation energy compared to published values for silicate dissolution