Metal-ion partitioning during low-temperature precipitation and dissolution of anhydrous carbonates and sulphates
Michael E. Böttcher, Martin Dietzel, 2011. "Metal-ion partitioning during low-temperature precipitation and dissolution of anhydrous carbonates and sulphates", Ion Partitioning in Ambient-Temperature Aqueous Systems, M. Prieto, H. Stoll
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Anhydrous carbonate and sulphate-bearing solids are formed on Earth under very different environmental conditions and are the most abundant constituents in so-called chemical sediments. At low temperatures, both biogenic and abiotic formation mechanisms are observed and their occurrence ranges from freshwater, via open marine to hypersaline aquatic environments. To understand individual mechanisms of mineral formation and the impact of boundary environmental conditions such as reaction temperature, composition of the aqueous mother fluid and precipitation rate, geochemical tracers such as trace metals or stable isotopes are needed. The thermodynamics of element partitioning in solid solutions-aqueous solutions (SS–AS) systems provides a frame for the evaluation of e.g. the impact of reaction kinetics, vital activity, or ionic strength. The presence of metastable phases may influence further reactions by providing the necessary reactants and surfaces for dissolution, adsorption, and re-precipitation reactions, thereby changing reaction kinetics in comparison to reactions in homogeneous solutions. The evaluation of trace-element substitution requires the combination of modelling approaches, in particular with experimental calibrations, but also empirical relationships. in this communication, the authors focus on selected binary carbonate and sulphate SS–AS systems at low temperatures. Systems involving the partitioning of Mg2+,Mn2+ and Sr2+ and others are discussed in more detail. They give a presentation of relevant current topics related to low-temperature metalion partitioning during complex precipitation and dissolution behaviour. The aim of this contribution is to stimulate future research in the highly relevant, low-temperature carbonate and sulphate SS–AS systems.
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Ion Partitioning in Ambient-Temperature Aqueous Systems
On the surface of the Earth, the intermingling of water and minerals gives rise to a diverse suite of reactions that determine the purity of water we drink, the fate of contaminants we emit, and the composition of minerals and biominerals that we use to interpret past environmental conditions from the sediment archive. Human societies have ubiquitous exposure to the outcome of these mineral—water reactions. Understanding in detail the ion partitioning in mineral—water interactions is of fundamental importance to geochemical studies and ultimately to society. The solid-solution properties of minerals are a significant part of the complexity, and also the importance, of these ion-partitioning reactions. Natural minerals always contain a certain proportion of trace elements in solid solution. These trace elements, precisely because of their rarity, often have a disproportionately large impact on living organisms as is the case for familiar toxic metals such as As and Cd. A clear understanding of ion partitioning behaviour is therefore essential for environmental objectives such as scavenging heavy metals from solution, remediating contamination in soils, or ensuring safe, long-term storage of anthropogenic CO2 or radionuclides in geological reservoirs. Materials science has also taken a new look at the role of trace-element and ion partitioning in regulating biomineralization. Finally, the last several decades have seen a surge in interest in reconstructing past climate and environmental conditions from the sediment archive. An accurate interpretation of ion partitioning is essential to the correct interpretation of records from diverse systems like stalagmites, corals, or shells of marine foraminifera. Given this wide range of applications for ion partitioning, it is fortunate that theoretical and thermodynamic frameworks for modelling ion partitioning have advanced significantly in the last decade. We believe that it is an opportune time to convene experts on ion partitioning from a range of perspectives, from theoretical to applied, to exchange knowledge across these topics and through this exchange, maximize the advances that have been made in the discipline. We are pleased to be able to convene these experts in person, at the European Mineralogical School in Oviedo in June 2010 to share these advances with each other and with the next generation of geochemists. It is our hope that this book will serve most crucially as a bridge through which researchers in one aspect of ion partitioning will be able to productively venture into complementary systems and models to better solve their research goals and perhaps be inspired with new research questions.