Genesis of Kaolinite from Montmorillonite by Weathering: Structural and Morphological Evidence of Transformation

Abstract

Kaolinite originating by the low-temperature subaerial transformation of montmoril-lonite occurs extensively in Tertiary clayey sands of central and northern Florida. It occurs in gray iron-stained weathering zones, as much as several feet thick, that transgress primary green clays of the Bone Valley and other formations over hundreds of square miles. Subjacent deposits of opal and chert, clearly post-depositional, are a widespread byproduct of the transformation.

Kaolinization begins as a hexagonal modification of the edges of montmorillonite flakes. X-ray study reveals that montmorillonite degrades and disappears up-section, with the appearance of the 7.2 kaolinite reflection and a very small 20–21 Å reflection. The altered clay loses silica and gains alumina and water.

The clay transformation apparently proceeds by intracrystalline leaching of tetrahedral silica layers by slightly acid ground water. Approximately regular interposition of resultant stripped packets within montmorillonite creates a regular 1:1 mixed-layered montmorillonite-kaolinite, which responds as montmorillonite does to glycolation and heating. The stripped kaoline-like packets develop hexagonal outgrowths of true kaolinite by lateral epitaxy. Nourished by the decomposing montmorillonite, the outgrowths enlarge and nucleate new vertical growth, observed as oriented crystallites, 40–50 Å thick.

Transformation of montmorillonite to kaolinite is a major geochemical process: creating goethite-stained kaolinitic sands of regional extent, liberating much silica for chert deposition and ground-water enrichment, and constituting an eluvial process that leads, in some areas, to residual quartz sand plains.