A mineralogical study of soil formation in four rhyolite-derived soils from New Zealand

Abstract

Summary In four yellow-brown earth and podzolised yellow-brown earth soils derived from rhyolite, much evidence concerning mineralogical changes attendant upon weathering and podzolisation has been obtained, and an explanation of the formation of the soils has been offered. Minerals varied in their powers of resistance to weathering in the order: oligoclase sanidine < mica (muscovitic) = chlorite < expanding micas (vermiculite and montmorillonite) < kaolin < gibbsite. Rhyolitic volcanic glass was less stable than sanidine, and quartz was at least as stable as gibbsite. There was some evidence to suggest that the order of resistance to podzolisation was almost the reverse of the order above, but again quartz was very resistant. Secondary silica crystallised as cristobalite appeared to be the only mineral formed by podzolisation. The sand fractions contained mainly primary minerals and indicated the relative stabilities of these minerals. The coarse and medium silts contained little other than quartz, but t...