This implies that the preferential lessivage of clay particles enriched in light 28Si and the resulting relative accumulation of primary clay-sized minerals in topsoil cannot be responsible for the on-going enrichment in light 28Si in the clay fraction. Indeed, the increasing enrichment in light 28Si in new tertiary minerals (tertiary kaolinite) in B horizons can only be related to a Si source progressively enriched in light 28Si over time.