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Diagenesis of silica minerals from clay minerals in volcanic soils of Mexico

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Indurated volcanic soils (tepetates) of the Mexican Altiplano display thick columnar horizons, hard laminar horizons, and grey mottles at depth. X-ray diffraction (XRD) studies show a relative enrichment in cristobalite vs. halloysite in the indurated plates of the laminar horizons and in the clay fraction of the mottles. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies of these two soil components have shown that they are composed of small tubes of halloysite in which numerous globular grains ~1 m in diameter are embedded. Based on the relative abundance of cristobalite in pedological features and on the spatial relations between successive mineral phases, we interpret the cristobalite as a transformation of halloysite with a transitional amorphous phase. In the globular grains, large platy 1:1 clay minerals undergo a progressive transformation into platy particles of opal-A and opal-C. These are in turn transformed into cristobalite without further major change in their shape and appearance, except for a higher electron density than opal and clay.
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Document Type: Research Article

Affiliations: 1: INRA, Science du Sol, Route de Saint-Cyr, F-78026 Versailles Cedex 2: IRD (ex ORSTOM), 32 avenue Henri 3: Varagnat, F-93143 Bondy Cedex, and 3 CIG, ENSMP, 35 rue Saint Honore ´, F-77305 Fontainebleau Cedex, France

Publication date: 2000-06-01

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