Thick iron oxide pans in soils of Taranaki, New Zealand

Abstract

Thick iron oxide pans are a distinctive feature of some soils in Taranaki, New Zealand, which occur on the ringplain, or on terraces of valleys draining the ringplain of Mount Egmont. The pans tend to form in the boundary area between layers of differing texture within the zone of water table fluctuations. The pans are indurated, brittle, and vesicular, and have a black or reddish brown appearance with a shiny black fracture. They are up to 50 cm thick and form lenticular deposits sometimes several metres across at depths ranging from a few centimetres to about 1 m. Analysis of seven samples of pan gave 34-45% elemental Fe, 3-5% A1 and 4-10% Si, consistent with about 55-70% iron oxides, together with entrapped and adhering soil particles. V and Mo are enriched in the pan samples and probably occur as anionic species strongly adsorbed on the iron oxide surfaces. X-ray powder diffraction, Moessbauer spectroscopy and acid-oxalate dissolution indicate that the dominant iron oxides present are goethite and ferrihydrite. The relative proportion of these two minerals varies widely without any noticeable change in the nature of the pan materials. Microstructures in one sample were examined by scanning electron microscopy. The pans are considered to have formed from the aeration of groundwaters (rich in ferrous ions) moving laterally through the soils. Such groundwaters are formed on Mount Egmont from the reaction of meteoric water, sometimes containing dissolved volcanic carbon dioxide, with ferromagnesian minerals. Positive tests for ferrous ions (using á,á'-dipyridyl as indicator) were obtained from groundwaters presently associated with the pans.