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Experimental constraints on indium transport in hydrothermal systems

Periodic Reporting for period 1 - HYDIN (Experimental constraints on indium transport in hydrothermal systems)

Reporting period: 2016-02-01 to 2018-01-31

Indium (In) is an element that has recently gained great economic importance due to its application in strategic energy and information technologies. Future In shortages projected by the EC Joint Research Council are due to insufficient exploration for In resources, reflecting the poor understanding of the hydrothermal ore-forming processes that result in economic enrichment of In. Key questions are the relative importance of different geologically relevant ligands for hydrothermal In complexation and the efficiency of different ore-deposition mechanisms for formation of economic deposits. Quantitative understanding of the solubility and transport of metals in hydrothermal fluids is central for process models of fluid-rock interaction and for predicting the formation of hydrothermal ore deposits. Predictive models based on experimental laboratory studies at elevated temperatures and pressures and numerical computer simulations are particularly powerful tools for understanding geochemical processes of hydrothermal ore-forming systems. Quantitative high-temperature data for the solubility and complexation of some ore metals such as Cu, Zn, Pb, Ag and Au have been obtained during the last two decades, but many important rare-metals (e.g. In, Ga, Ge, Sc, Rare Earth Elements, Nb, Ta) have not been experimentally studied and their behavior in hydrothermal systems can currently not be modeled. With the new solubility and spectroscopy experiments on fluoride and chloride complexes of indium generated by this project, we provide the relevant input data that make it possible to create predictive In distribution models in ore deposits. The strong differences in complexation behavior across the hydrothermal temperature range of 200-400 °C relevant for such systems explains the large variability of different In deposit styles. The new data will help secure economically feasible In supply in the future for high tech applications such as smart phone displays and solar panels.
Solubility experiments at hydrothermal conditions were performed in order to obtain indium hydroxide, fluoride and chloride complexation constants. For this purpose a new experimental laboratory was set up at University of Helsinki. A total of 117 individual experiments were conducted at 150, 200, and 250°C and for a range of different hydrofluoric and hydrochloric acid concentrations and different pH. The results are being processed using a novel approach for simultaneous regression of thermodynamic datasets. Data analysis shows that at 150°C, pH exerts a strong control on the solubility of In(OH)3 with solubility being lowest at low pH. This demonstrates that solubility is controlled by hydroxide complexes. At 250°C, In solubility increases with chloride concentration, implying that chloride complexes are controlling In solubility at this temperature. Synchrotron X-Ray fluorescence (SXRF) measurements at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, were used to obtain Extended X-Ray Fine Structure (EXAFS) spectra of indium in fluoride and chloride bearing solutions between 30 and 400°C. The data show that fluoride complexes of indium are only stable at temperatures below 200°C and that chloride complexes become increasingly stable at higher temperatures. A continuous transition from fluoride dominated (at 30°C) to chloride dominated (at 400°C) indium complexation was observed in a series of experiments with mixed fluoride-chloride solutions. The results of this study are currently being prepared for publication in peer reviewed journals. The outcomes of the solubility and EXAFS experiments will be summarized in a publication in a leading international geochemistry journal. A final synthesis and integration with application to natural data will be covered in a publication in a leading mineral deposit journal. The project was presented to the general public at Researchers Night 2016 at the Heureka Science Centre in Vantaa, Finland. The EXAFS results were presented at the European Geosciences Union General Assembly 2017 in Vienna. The results will also be presented in a series of invited talks at leading European research and technical universities in summer 2018.
The solubility and spectroscopic data show that the In3+ cation exhibits an unusual behavior in aqueous solutions because it predominantly forms complexes with small anions at ambient temperatures but with larger ones above 200°C. This means that the chemical character of In changes from that of a hard acid in the Lewis-Pearson Acid Base Concept, to that of a soft acid with increasing temperatures. Because the transition temperature is crossed in the typical temperature range of hydrothermal ore-forming systems, this has far-reaching consequences for hydrothermal transport and precipitation of In in natural environments. Depending on the chemical composition and pH, the solubility of indium will either decrease or increase during cooling upon ascent of a fluid. This is a highly unusual behavior for metal cations and we interpret this as the key reason why indium is heterogeneously distributed in economic deposits. Taking advantage of the new data from this project, we can now for the first time predict indium distribution in ore deposits based on models of temperature and fluid composition. The latter can be gained from studies of fluid inclusions, small samples of the solutions from which the hydrothermal ore deposits formed. A better understanding of the controls on In distribution and enrichment in economic ore deposits is essential for securing future supply of this critical element for our high-technological society. We must remember that every time we use a smart phone screen, we look through a transparent layer of indium-tin-oxide conductors that makes it possible that the device is small enough to fit in our pocket.