Recent Advances in Formation Mechanisms of Minerals in Precipitates, Salt Efflorescences and Sulfidic Materials in Acid Sulfate Weathering Environments.
Robert Fitzpatrick, CRC-LEME/CSIRO Land and Water, Waite Road, Urrbrae, Adelaide, Australia
The objective of this paper is to discuss the occurrence and formation of several new assemblages of sulfate-containing evaporite minerals, oxyhydroxysulfate minerals and sulfides in previously unreported natural soil environments. The significance of these minerals is that they are environmental indicators because they are formed by the unique soil and ground water geochemistry of the region. 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- and sulfide- containing minerals in geo-chemically variable Acid Sulfate Soils (ASS). Mechanistic models use the toposequence approach, which integrates pedological, mineralogical, hydrological, biogeochemical, geological, climatic and land-use information. The paper highlights case studies of evaporite, oxyhydroxysulfate and sulfide minerals in various types of ASS in Australia and Iraq. 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. These solutions are able to scavenge anomalous concentrations of elements such as Cu, Pb, Zn, from mineralized zones in bedrock. Sulfidic materials may contain two types of iron sulfide: pyrite and Fe monosulfides (greigite or mackinawite). In the vicinity of mineralized zones in bedrock, sulfidic materials may contain Cu, Pb and Zn sulfides, native gold, barite and Mn oxides (containing minor Co, Zn and I). These minerals tend to be intimately associated with, and incorporated in, organic matter. In particular, Zn and Pb sulfides tend to occur in very fine (<1 µm diameter) spherical grains precipitated via biomineralization processes. The composition of the Zn sulfide in sulfidic material is relatively Fe-poor, in contrast to the relatively Fe-rich sphalerite from the nearby primary mineralized zones. 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 that form in varied physico-geochemical environments: (i) sideronatrite, tamarugite, copiapite, hexahydrite, 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, wattevilleite, glauberite, gypsum, konyaite, thenardite, mirabilite, barite, 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.