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Characterization of Aluminum Interlayers in Soil Clays¹

ABSTRACT

The amounts of Al, Fe, Mg, K, and Si extracted by sodium citrate from 10 soil clays containing various amounts of chloritized vermiculite are related to changes in cation-exchange capacity (CEC), intensity of the 14A spacing and sample weight loss. Those soils containing abundant interlayers underwent the largest changes in CEC, contained the most citrate-soluble Al and lost the most weight. Although most of the change in CEC and intensity of the 14A spacing occurred in the first 2 hours, Al was steadily extracted from sources of similar solubility for at least 6 hours. Even though the increase in CEC was well correlated with the total Al extracted, it appears that Al interlayers exist as "atolls," and that not all citrate-soluble Al is contained in interlayer positions. No evidence for the presence of Fe interlayers was found; rather, Fe and Mg appear to exist as contaminants in citrate-soluble Al compounds. A weathering sequence of illite-vermiculite-chloritized vermiculite-gibbsite is proposed for these acid soils. Thus, abundant Al interlayers and low CEC are among the most prominent features of highly weathered acid soils.

¹ Contribution from The Connecticut Agr. Exp. Sta., New Haven.

² Associate Soil Chemist.

Received for publication November 9, 1964. Accepted for publication February 10, 1965.