Transformation of Montmorillonite to Kaolinite during Weathering

Abstract

Extensive deposits of kaolinite in Florida are formed by transformation of montmorillonite during low-temperature supergene weathering. The transformation occurs by intracrystalline leaching of interlayer cations and tetrahedral silica layers. Interposition of stripped layers within montmorillonite creates a regular 1:1 mixedlayered montmorillonite-kaolinite, a new clay structure. Kaolin-like layers are nourished by lateral epitaxy, as the iron-rich montmorillonite decomposes. Hexagonal outgrowths of new kaolinite develop at the edges of montmorillonite flakes and nucleate new vertical growth. Kaolinitic sands impregnated with goethite are ultimately formed, and the released silica enriches groundwater and forms secondary chert.

Synthesis of Mixed-Layer Kaolinite/Smectite

“…Drits and Sakharov (1976) suggested simultaneous crystallization of both types of layers due to oscillatory changes of chemical parameters in the microenvironment surrounding the crystallizing particle. The data presented in this paper cannot be explained by the mechanism proposed by Altschuler et al (1963); an excess of amorphous silica should be present if one of the tetrahedral sheets was removed. On the other hand, the mixtures of mixed-layer kaolinite/smectite and discrete smectite in the run products do not necessarily imply the crystallization of mixed-layer kaolinite/smectite from solution.…”

“…Previously, the transformation to a mixed-layer through the removal of one tetrahedral sheet from a smectite layer was advocated by Altschuler et al (1963). Drits and Sakharov (1976) suggested simultaneous crystallization of both types of layers due to oscillatory changes of chemical parameters in the microenvironment surrounding the crystallizing particle.…”

“…The existence of mixed-layer kaolinite/smectites in nature was first suggested by Sudo and Hayashi (1956) and Altschuler et al (1963). In a sense, these minerals were rediscovered in the Tertiary clays of Yucatan by Schultz et al (1971) and in hydrothermal deposits of Lower Silesia by Wiewifira (1971Wiewifira ( , 1973.…”

“…In most of the published cases it can be shown that kaolinite/smectite forms at the expense of smectite (Altschuler et al, 1963;Thiry, 1973;Drits and Sakharov, 1976). A study was undertaken to simulate the alteration of smectite towards kaolinite in the laboratory, and preliminary results are reported in this paper.…”

Physical, Chemical and Engineering Properties of Residual Limestone Soils and Clays.

“…Because some of the elements are, in fact, soluble in dilute acid solutions, the fact that they do not decrease as the soils mature must indicate that whatever mineral transformations are taking place do not, as some believe, involve the dissolution of the unstable mineral followed by re-precipitation to form a new stable phase (e.g., illite altering to kaolinite). Rather, the change must occur by cation stripping of the unstable mineral followed by structural rearrangement to form the new mineral (see Altschuler, et al, 1963). …”