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

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. h alloysite in the indurated plates of the laminar horizons and in the clay f raction of the mottles. Transmission electron microscopy (TEM) and high-res olution TEM (HRTEM) studies of these two soil components have shown that th ey are composed of small tubes of halloysite in which numerous globular gra ins similar to 1 mu m in diameter are embedded. Based on the relative abund ance of cristobalite in pedological features and on the spatial relations b etween successive mineral phases, we interpret the cristobalite as a transf ormation 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 rum transformed in to cristobalite without further major change in their shape and appearance, except for a higher electron density than opal and clay.