Reaction mechanisms for the reaction smectite ^rarr mixed-layer illite/smectite are proposed based on experimental results. Pure smectite (100% expandable) in NaCl-Na₂CO₃ solutions was heated in sealed gold capsules to 180 and 350°C for periods of 7 and 28 days. The solutions used for this initial hydrothermal treatment were kept K ± free to prevent the formation of non-expanding illite layers, a condition which would have made it difficult to characterize the exchange properties of expanding layers undergoing transition.

A series of cation exchange experiments using chloride solutions of Na⁺, K⁺, and Mg⁺⁺ were carried out at 25°C to determine the effect of the heat treatment described above on K± selectivity and total cation exchange capacity. Data obtained from these experiments show that the hydrothermally treated clays become less selective for K⁺ with increasing temperature and length of run. Data also show that the total CEC appears to decrease with increasing temperature and length of run. We suggest that this apparent decrease in total CEC as well as the difference of K ± selectivity is caused by the inability of the exchange cation (Ba⁺⁺ in these experiments) to replace tightly held A1, which diffused to exchange sites from etrahedral and octahedral positions when the clays were heated.

These results lead us to propose that a smectite-interlayer Al-hydroxy complex may be an important intermediate in the smectite^rarrmixed-layer illite/smectite transformation often observed in buried pelitic sediments. Further study of the rate and mechanisms involved in the formation and ultimate destruction of the Al-hydroxy interlayer material should be

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instructive in predicting the behavior of fluid movement during burial diagenesis/metamorphism of pelitic sediments.

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