Hydrous iron oxide minerals with short range order deposited in a spring/stream system, Tongariro National Park, New Zealand

Abstract

An iron-rich deposit occurs on the bed of the Waiuku Stream and one of its tributaries, on Mt Ruapehu, New Zealand. The upper limit of the deposit is a spring which vents in the bed of the tributary. Isotope ratios, 18O/16O, D/H, and 13C/12C, indicate that the spring water is largely meteoric in origin, from a catchment at higher altitude, and that the dissolved carbon dioxide has a volcanic source. Dissolved solids appear to be derived from acid attack on andesite. Ferrous iron (30 g m-3) in the spring water is oxidized on reaching the surface, at a rate which can be accounted for abiotically, forming the ochrous deposit. The major components of the deposit are Fe (40-50%), Si (5-10%), H2O(+) (11-13%), and H2O(-) (14-23%), and surface areas of samples lie in the range 120-240 m2 g-I. Electron micrographs show globular aggregates of indistinct, approximately spherical, particles with diameters about 5-10 nm. This information, together with X-ray diffractograms, i.r. and differential thermal analysis patterns indicate that the deposit is a hydrous iron oxide (more correctly, oxyhydroxide) with short range order. Most of the samples have characteristics close to those of material presently known as protoferrihydrite, though one is closer to ferrihydrite. The deposit readily adsorbs silica and phosphate from the stream water.