Estimation of the amounts of allophane and other materials in the clay fraction of an Egmont loam profile and other volcanic ash soils, New Zealand

Abstract

The allophane content of the clay fraction of six samples taken at different depths in an Egmont loam profile and samples from three other volcanic ash soils has been determined by acid-oxalate dissolution and infrared spectroscopy and the results from the two methods were in good agreement. The tephra that accumulated during 3500-10000 years B.P. (30-80 cm depth) in the Egmont profile had the highest allophane content (about 70% of the clay fraction). The allophane was identified as the proto-imogolite form of allophane. The allophane content of the clay fraction of the top 10 cm of the profile was lower than that of the underlying horizons, and the clay fraction also contained significant amounts of glass, laminar opaline silica, imogolite, and halloysite. A sample from the lower unit of the profile (below 80 cm), derived from tephric loess, contained about 50% allophane in the clay fraction, together with halloysite, gibbsite, chlorite and quartz. Halloysite was recognized in all the samples from infrared absorption bands at 470, 1035 and 1100 cm-1. These bands were enhanced and the other halloysite bands were observed after allophane was removed by selective dissolution in acid-oxalate. Chlorite appeared to be a common minor component of the clay fractions and probably has an aeolian origin.