Metal-ion partitioning during low-temperature precipitation and dissolution of anhydrous carbonates and sulphates

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 reprecipitation 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 metal-ion 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.