ALLOPHANE AND HALLOYSITE FORMATION IN A VOLCANIC ASH BED UNDER DIFFERENT MOISTURE CONDITIONS

Abstract

We examined a rhyolitic volcanic ash bed deposited 20000 years ago in different leaching environments in New Zealand. When winter leaching was greater than 250 mm a year, the ash contained 10 to 15% allophane; as leaching increased from 250 to 600 mm a year, the A1/Si ratio of the allophane increased from 1.2 to 2.0. When the leaching was < 250 mm a year and water movement was somewhat restricted, the allophane content of the bed was very low, and halloysite was the predominant clay mineral. The results support a hypothesis that allophane with A1/Si ratio close to 2 forms in strongly leached environments, possibly because Si in the soil solution is low; halloysite forms in weakly leached environments, possibly where Si in soil solution is more concentrated.