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

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

^* E-mail: elsass{at}versailles.inra.fr

(Received 11 April 1997; revised 31 August 1999)

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 µm 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.

KEYWORDS: opal, cristobalite, volcanic soils, induration, silicification, X-ray diffraction, electron microscopy

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