IMOGOLITE OF CHILEAN VOLCANIC ASH SOILS

Abstract

Volcanic ash soils were collected from 8 sites which distributed from around 35°30' to 41° of the south latitude in the Central Valley of Chile, and their clay minerals were studied mainly by x-ray, differential thermal and electron microscopical methods. The soils examined could be classified into three large groups according to the major clay minerals; allophanic, metahalloysitic, and allophane-metahalloysitic soils.
The allophanic soils correspond to “trumao” soils derived from young volcanic ashes, and the metahalloysitic soils to “rojo arcilloso” soils derived from old volcanic ash materials. The allophane-metahalloysitic soils are morphologically “trumao” soils, but their parent materials would be mixtures of both young and old ashes.
Besides the major clay minerals, the allophanic soils contain imogolite, kaolinite, chlorite, gibbsite, irregularly interstratified chloritevermiculite and occasionally smectite, vermiculite and illite. In contrast to those soils, the minor clay minerals of the metahalloysitic soils are halloysite, chlorite and irregularly interstratified minerals of halloysitemetahalloysite and vermiculite-chlorite. They sometimes contain illite, vermiculite, smectite, gibbsite and irregularly interstratified smectitevermiculite. The clay mineral assemblage of allophane-metahalloysitic soils is similar to the metahalloysitic soils excepting abundant allophane.
Imogolite in the soils was identified by its characteristic x-ray diffractions, endothermic reaction, and fibre-like shape, but its particles are rather short in length. Imogolite seems to occur very widely in the “trumao” soils derived from young volcanic ashes in Chile.