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Soil and water dispersed size separates from the B and C horizons of a Taupo sandy silt, a Vitric Andosol (Vitrandept) from New Zealand (age 1700 years), were examined by a range of chemical and instrumental techniques. Results showed that the clay fractions (< 2 μm) contained both comminuted glass and a weathering product that had many of the properties of synthetic aluminosilicate gels. These two components differed markedly in chemical composition and properties. The SiO2/Al2O3mole ratio of the gel phase was less than 2.0, while that of the fine glass exceeded 8.0. Only the gel phase, which contained appreciable six-fold co-ordinated Al, showed a high temperature exotherm on DTA. Increasing accumulation of the more aluminous gel phase with time would account for the weathering scheme: