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

Open Access
Publisher Website
Google Scholar
Abstract
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 the fine silts contained both primary and secondary minerals and indicated the relative stabilities of the most stable primary minerals and the least stable secondary minerals. The clay fractions contained mostly secondary minerals, were the most susceptible to katamorphic changes, and indicated the relative stabilities of secondary minerals. Koikoi sandy loam was formed under low to moderate weathering anod low podzolisation on a steep slope. It contained nearly equal amounts of mica, kaolin and quartz in its clays. Maungarei clay loam was formed under strong weathering and low podzolisation and its clay was predominantly kaolin. Pukenamu silt loam was formed under strong weathering and moderate pcdzolisation and had definite eluvial and illuvial horizons. Its clays were predominantly kaolin and quartz with quartz concentrated in the eluvial horizons. Parahaki sandy loam was formed under strong weathering and strong podzolisation, and showed marked differentiation into eluvial and illuvial zones. Quartz was the dominant mineral in the eluvial horizons but gibbsite and kaolin were dominant in illuvial horizons. Anatase was concentrated in one illuvial horizon. The C horizon of this soil contained an unusual “stuffed” cristobalite with a high cation exchange capacity and high water holding capacity.