- Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT) (2)
Iron in oxides, silicates and alloys under extreme pressure-temperature conditions
- (1) There is a general agreement, that magnesium silicate perovskite (Pv) comprises around 80 vol% of the Earth's lower-mantle, making it by volume the most abundant mineral in our planet, and and there is no doubt that Pv in the mantle contains Fe and Al. However, the exact concentrations are unknown, as well as the effect of pressure on physical properties of Pv at conditions of Earth lower mantle. In our study we investigate Pv with one of the less explored substitution Mg2+A+Si4+B→Fe3+A+Al3+B. Here we explore as a function of pressure and temperature the crystal structure of the material, the distribution of chemical elements between different crystallographic sites and the evolution the spin state of ferric iron, as one of crucial parameters determining electrical and radiative conductivity of the Earth's lower mantle. We perform single-crystal x-ray diffraction on magnesium silicate perovskite with the composition Mg0.63Fe0.37Si0.63Al0.37O3 (MgFeAlPv) using a combination of in-situ diamond anvil cell technique and laser heating in order to simulate the extreme conditions of the Earth's lower mantle. We provide a complete description of the behavior of MgFeAlPv in terms of crystal structure and ferric iron occupying its dodecahedral (A-)site. We observe no spin transition of ferric iron at A-site, confirming theoretical predictions and recent experimental observations. However, even upon heating MgFeAlPv samples to 1800 K at ~78 GPa we see no indication of a spin crossover or a pressure/temperature induced redistribution of ferric iron and aluminum between the different crystallographic sites as suggested previously. We combine these data with high pressure-high temperature measurements to obtain a thermal equation of state. (2) As a model Fe-O system, magnetite is a mixed valence iron oxide incorporating both ferric and ferrous iron. Being essential part of some sedimentary (banded iron formations) and igneous rocks, magnetite can be subjected to high pressure in natural systems, for instance, during subduction of oceanic crust, or during serpentinization (metamorphic reaction). In order to shed light on the complex physical properties of magnetite under compression we conducted a combined single crystal x-ray diffraction and Mössbauer spectroscopy at pressures below 25 GPa. We find no evidence for the transition from inverse to normal spinel in magnetite. Analyzing the collected Mössbauer data, we show that a high spin – intermediate spin transition cannot occur in magnetite in the pressure range of 10-20 GPa, and finally, based on a careful analysis of the data and results reported in the literature, we provide a model consistently describing the behavior of electronic and magnetic properties of magnetite in terms of a gradual charge delocalization induced by pressure. (3) Our study of wüstite (FexO) is focused on the high pressure – low temperature phase diagram of the Fe-end member in the (Mg,Fe)O system. We perform high resolution neutron diffraction experiments in order to investigate the low temperature phase diagram of Fe0.925O and Fe0.94O. We determine the critical temperatures of antiferromagnetic ordering (the Neél temperature TN) and structural transitions (TS) of the two compounds. We report divergence of TN and TS as a function of pressure. We argue that a modification of the defect structure in wüstite can be invoked explaining the drastically different response of Fe0.925O and Fe0.94O to compression. With that we show that although ferric iron is a minor structural component of wüstite, it is an essential component of defect structures and induces profound effects on the low temperature phase diagram of wüstite. (3) We investigate effect of pressure (P) on the elastic and electronic properties of Fe, Fe0.9Ni0.1 hcp phases below 70 GPa. After processing our experimental data, we report a gradual decrease in the ratio of the hcp lattice parameters c/a for Fe in the pressure range below 45-50 GPa, and a non-linear behavior of Mössbauer isomer shift for hcp phases of pure Fe and Fe0.9Ni0.1, suggesting an isostructural transition in these phases. We investigate paramagnetic hcp Fe under compression by employing state-of-art calculations (LDA+DMFT) and including many-body correlation effects. Based on the results of the calculations, we predict an electronic topological transition (ETT). After comparing data on materials with already known ETT with our observations and theoretical predictions, we conclude that results obtained from the independent experimental measurements can be explained in the framework of an ETT. (4) The development of a portable laser heating system was a necessary requirement for our work done on minerals at conditions of Earth’s lower mantle in general, and for the study of magnesium silicate perovskite containing iron and aluminum in particular. The main advantages of the system developed are compactness, versatility for different in-house and synchrotron based techniques, including high pressure measurements of resistivity, Raman spectroscopy, energy and time-resolved Mössbauer spectroscopy, powder and single crystal x-ray diffraction, nuclear inelastic x-ray scattering, and x-ray absorption. These advantages, the low times of assembly, stable and homogeneous conditions for heating, in-situ measurement of sample temperature, as well as the direct visual control over the heating area distinguish our system from similar, but bulkier devices.
Investigations towards a better understanding of arsenic-sulfur speciation in aquatic environments: Formation, stability, structural characterization, and conflicting analyses
- Arsenic is a widespread contaminant of global concerns due to its neurotoxicity and carcinogenicity. Particularly critical is the speciation of arsenic, influencing its mobility, toxicity and retention capability. Recently, it was analytically proven that arsenic-sulfur (As-S) species play a dominant role for arsenic cycling in sulfidic systems. The geochemistry of As-S species is not well investigated, yet, and especially the nature of these species, thioarsenites vs. thioarsenates, has been under intense debate. The major objective of the present PhD work was to improve the current knowledge about As-S species by structural characterization, investigations of their occurrence, formation and transformation, and evaluation of the analytical techniques, X-ray absorption spectroscopy (XAS) and ion chromatography coupled to ICP-MS (IC-ICP-MS). By XAS it was shown that under strictly anoxic conditions thioarsenites form in arsenite-sulfide mixes with sulfide (SH-) excess and as co-occurring intermediates during acidic transformation of thioarsenates. Thioarsenites can be specified as highly labile, converting rapidly to thioarsenates in the presence of traces of oxygen, e.g. during standard IC-ICP-MS analyses. Excess hydroxide (OH-) either due to high pH or sample dilution in ultrapure water inhibits the formation of thioarsenites by SH--OH- competition. These facts make the current IC-ICP-MS method even under anoxic conditions unsuitable for thioarsenite analyses. However, thioarsenites were shown to be necessary intermediates for formation of thioarsenates. Thioarsenates determined in natural oxic systems are thus most likely the product of rapid in-situ thioarsenite oxidation. Direct thioarsenite determination is currently only possible by XAS with a limitation on > 5 mM-solutions for structural evaluations. The characteristic coordination and bond length (RAs-S 2.23-2.28 Å) makes thioarsenites distinguishable from thioarsenates (RAs-O 1.70 Å, RAs-S 2.13-2.18 Å). The individual thioarsenates are distinct in their coordination and absorption edge energies, successively decreasing about 1 eV per sulfur atom. Generally, the absorption edge energies decrease in the order arsenate > thioarsenates > arsenite > thioarsenites. This primary XAS-dataset enables the evaluation of (thio)arsenites and (thio)arsenates in mixed solutions. Despite the greater stability of thioarsenates vs. thioarsenites, they also have been shown to transform under certain conditions. Upon acidification they convert to thioarsenites (anoxic) or arsenite (oxic) with subsequent As-S precipitation. The presence of FeII in anoxic solutions or heating (80 °C) results in their decay to substantial amounts of arsenite. Thioarsenates are also easily oxidized by synthetic oxidants, air purging or naturally along hot spring drainage channels. For trithioarsenate, the major species of alkaline hot springs in Yellowstone National Park, two transformation processes have been identified: successive ligand exchange to arsenate, observed naturally and by using a strong oxidant, and the decay to arsenite (and trithioarsenate) in natural systems and under moderately reducing conditions. However, transformation under natural conditions was up to 500 times faster and is likely catalyzed by Thermocrinis spp.. Naturally important are also processes promoting mobilization or immobilization of arsenic from and at mineral surfaces. Arsenopyrite and orpiment belong to the most abundant (Fe-)As-S minerals with particular importance as host rocks for gold refractory. Oxidative leaching of both minerals yielded up to 50% thioarsenates. The release of thioarsenates from orpiment, at pH 7 and 12, is possibly caused by thioarsenite oxidation. Contrary, physisorption of OH- is the proposed mechanism for arsenopyrite with thioarsenate formation only at highly alkaline pH. The immobilization of monothioarsenate by sorption on ironhydroxide was less effective and kinetically slower compared to arsenate and arsenite. The presence of iron in As-S systems was hitherto considered to counteract thioarsenate occurrence. This was refuted by finding up to 17% thioarsenates in Czech spring waters. However, those Fe As S systems are a challenge for sample preservation. While acidification results in As-S precipitation and thioarsenate transformation, flash-freezing as preferred for thioarsenates induces ironhydroxide precipitation. An anoxic gas headspace, a strong matrix and an organic solvent supported the stability of pure thioarsenate solutions, whereas in the presence of iron a combination of EDTA-addition and cryo-preservation is required. Overall, the present PhD thesis reveals the importance of thioarsenites and thioarsenates for arsenic cycling. The results significantly increase the present knowledge on As-S geochemistry and help to define potential for future studies.