INTERSTRATIFIED KAOLINITE-SMECTITE: NATURE OF THE LAYERS AND MECHANISM OF SMECTITE KAOLINIZATION

Abstract

This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratified clay minerals. We examined the <2 m or <0.2 size fraction of interstratified kaolinite-smectite (k-s) formed by hydrothermal and hydrogenic alteration volcanogenic material from a tortonian clay deposit (almer, spain), weathered eocene volcanic ash (yucatan, mexico), jurassic bentonite (northamptonshire, england). the methods used were x-ray diffraction analysis (xrd) random oriented preparations, thermogravimetry, chemical analysis, 29Si MAS nuclear magnetic resonance. The proportions of kaolinite and smectite in K-S (%K) were determined by fitting the XRD patterns of ethylene-glycol-saturated samples with patterns calculated with the NEWMOD computer program. The obtained range of compositions is 0-85%K. A comparison of the results from the various techniques showed non-linear relationships, indicating that the layers in K-S are complex and hybrid in nature. The smectite-to-kaolinite reaction is a solid-state transformation proceeding through formation of kaolinite-like patches within the smectite layers. The process consists of several non-simultaneous stages: (1) removal of parts of the tetrahedral sheet, resulting in formation of kaolinite-like patches; (2) layer collapse to ~7 Å where the kaolinite-like patches are sufficiently large; (3) Al for Mg substitution in the octahedral sheet, simultaneous or slightly delayed with respect to layer collapse, causing a layer-charge decrease and loss of interlayer cations; (4) Si for Al replacement in the tetrahedral sheet and further loss of interlayer cations. Iron remains in the kaolinite or is lost at the latest stages of the process.