A hydrofluoric acid dissolution method for determining volcanic glass content of tephra derived soils (andisols)

Abstract

The determination of volcanic glass content is essential for the classification of many Andisols in Soil Taxonomy. Previously, polarizing microscope and heavy liquid methods have been used to estimate glass content, but we suggest here an alternative method based on gravimetric analysis of glass-bearing samples before and after hydrofluoric acid (HF) dissolution treatment. A series of Bw and C horizons of weakly developed buried paleosols derived from post-c. 15 ka rhyolitic tephra deposits in North Island, New Zealand, was subjected to the HF treatment and glass contents calculated. Our results, corroborated by qualitative examination using a polarizing microscope, indicate that the HF dissolution method for these materials is simple, fast, and accurate, and, with further testing, could be applicable to all types of glass. Slight modifications to the method are probably needed where glass compositions or primary mineralogical assemblages vary significantly between samples. As well as being potentially useful for characterizing and defining Andisols, the method is likely to be applicable wherever estimates of the volcanic glass content of soils, paleosols, or tephra deposits are required.