MINERALOGY AND RADIOISOTOPE RETENTION PROPERTIES OF A CHRONOSEQUENCE OF SOILS DEVELOPED IN BASALTS OF VICTORIA, AUSTRALIA

Abstract

SummaryThe mineralogical composition and retention properties for radioisotopes (20Sr and 137Cs) of soils developed in five basalt flows of age varying from 6000 years to about four million years occurring in western Victoria were investigated. The trend of mineral weathering has been almost exclusively to amorphous material, kaolinite‐plus‐halloysite, and chlorite, the more soluble products of weathering having been removed. The most significant changes in clay mineralogical composition with time are the progressive decrease in the Si02/Al2O3 molar ratio of the amorphous material in the clay fraction of the surface horizons, from an initial value of approximately 4 to values of approximately 2, and the progressive increase in the amount of kaolinite‐plus‐halloysite, both in the topsoil and at depth, with age of the basalt flow. The amount of kaolinite plus halloysite increases from approximately 20 per cent of the clay of soil developed in the basalt flow 6000 years old to approximately 50 per cent of that of soil in basalt about four million years old. Evidence for the presence of halloysite was obtained by electron microscopy studies. The amorphous material and chlorite contents, each of which constitutes between 20 and 50 per cent of the clay fractions, decrease concurrently with the increase in kaolinite‐plushalloysite content.Fixation of Sr by whole soil samples was controlled by the organic matter and free iron oxide contents rather than by the mineralogy of the samples. A high proportion of the added Cs was sorbed by whole soil samples. Much of the sorbed Cs was not readily replaced by CaCl, washings but was replaced in part by subsequent washing with NaCl of pH 5.3 and almost entirely by subsequent NH4Cl washings. Much of the Sr and Cs deposited on these soils by rainfall and dry fall‐out would be sorbed; the ease of replacement suggests that these elements would be available for further movement through the food chain.