This paper provides an overview of our approach and procedures to construct appropriate 3D and 4D mechanistic models of soil-regolith and water processes that explain and predict processes giving rise to different assemblages of sulfate-containing evaporite minerals in geo-chemically variable acid sulfate soils (ASS). The paper highlights case studies of evaporite minerals in various types of ASS in Australia and Iraq.

Mechanistic models use the toposequence approach, which integrates pedological, mineralogical, hydrological, biogeochemical, geological, climatic and land-use information. The combination of seawater or saline groundwaters enriched in sulfate (with other elements sourced from mineralised zones) seeping through soils, anaerobic conditions and organic carbon in saturated soils yield pyrite-enriched sulfidic material containing pyrite framboids through anaerobic bacterial reduction of sulfate. When sulfidic materials are eroded and exposed to air, pyrite is oxidised producing sulfuric acid with mineral dissolution followed by precipitation of the following sulfate-containing minerals in varied environments: (i) sideronatrite, tamarugite, copiapite, pentahydrite, starkeyite, bischofite, bassanite, carnallite, rozenite, barite, halite and gypsum in sandy sulfuric horizons with pH <3.0; (ii) natrojarosite, jarosite and plumbojarosite in clay-rich sulfuric horizons with pH 3.5-4, (iii) eugsterite, bloedite, thenardite, glauberite, gypsum, thenardite, mirabilite, schwertmannite, lepidocrocite, akaganéite and colloidal poorly crystalline/pseudoboehmite-like (white) precipitates in sulfidic materials with pH >5. These minerals may range in morphology from thin, powdery, and very transient efflorescences to thicker, more persistent, soil-cementing crusts. Formation of these complex salts of sulfates of Fe, Al, Na, Pb, Ca, As, Zn, Mg, jarosites, oxyhydroxysulfates and oxyhydroxides are indicative of rapidly changing local environments and variations in redox, pH and rates of availability of S and other elements. As such, these evaporite minerals are indicators of soil-water processes operating in specific landscapes. A detailed understanding of these minerals and biogeochemistry in ASS has revealed important dual applications for land management and mineral exploration.