Occurrence and environmental significance of sideronatrite and other mineral precipitates in Acid Sulfate Soils

Abstract

This study documents the first occurrence of metavoltine in Australia and the widespread occurrences of sideronatrite and tamarugite in Acid Sulfate Soils (ASS). We interpret the occurrence of these soluble salts to represent changing surface and ground water tables, which are linked to the lowering of water levels in the River Murray and Lower Lakes where capillary action, combined with subsurface evaporation, has concentrated Fe-Al-Na-Mg sulfates, especially in summer or during dry periods. During winter rainfall wet/drying events in the sandy sulfuric materials on exposed beaches, soluble white sulfate-containing evaporite minerals comprising pickeringite-halotrichite, redingtonite, hexahydrite and epsomite precipitate as micron thick layers on the soil surface. Sideronatrite and tamarugite precipitate within yellowish-green friable 2 to 5 mm thick crusts on the soil surface. Sideronatrite (large platelets) is derived from the oxidation and dissolution of the sulfide framboids in sulfuric materials (< pH 2.5). Surrounding some of the crusts where water temporarily leaches and ponds, sideronatrite dissolves and re-precipitates as schwertmannite within orange coloured patches. These mineral precipitates play important roles in the transient storage of components (Fe, Al, Na, Ca, Mg, Cl, Sr and SO4), which may also dissolve to contribute to the formation of saline monosulfidic black ooze in wetter soils (e.g. adjacent to the lakes). These salts are likely to form if water levels decrease and have the potential to become a problem during re-flooding if not managed properly. Salt efflorescences also have potential for aerial transport and to be dissolved in water. There is a need to prevent stock from ingesting these salts (similar to Epsom salts) because this is likely to lead to scouring in sheep and cattle. Magnesium salts are toxic when ingested in high levels.