Mineralogy of soils from the holocene volcanic area of Southern Australia

Abstract

The mineralogy and chemical composition of the soils developed on Holocene basaltic ash deposits in southern Australia have been examined. The bulk of the clay-sized material in all samples was made up of a morphologically continuous gel incorporating small amounts of crystalline primary minerals and layer silicates. Fibrous allophanes were present, but no imogolite. Weathering of the primary material is accompanied in all cases by an increase in the proportion of hydrous oxides (from about 15 to 18%) and a reduction in the Si/Al ratio, irrespective of the age of the deposit. A slight predominance in silicon occurred in the clay samples, but this decreased to a Si/Al ratio of around 1.0 with increasing age and depth of the samples. The variations in the composition of the ash appear to control the formation of subsequent clay types, and no single weathering sequence describes the mineral alterations in the soils of even so restricted a geological and climatic region.