Development and Application of Space- and Time-Resolved Hyperspectral Raman Imaging to in situ Study Mineral- and Glass-Water Reactions

• Applicant: Professor Dr. Thorsten Geisler-Wierwille
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 389799517

Solid-water reactions are often characterized by the formation of surface alteration layers that complexly affect the dissolution behaviour of the solid. Despite numerous sophisticated experimental studies, there is still an intensive debate about how these alteration layers form and about the rate-limiting reaction step. New insights into the mesoscopic partial reactions underlying their formation can be gained from confocal hyperspectral Raman spectroscopy (CHRS). This technique allows real-time imaging of moving reaction interfaces during fluid-mediated replacement reactions at the micrometer-scale and under elevated temperatures with the use of fluid cells (FC), omitting the imperative to interrupt the replacement reaction. FC CHRS thereby can provide a wealth of information such as, e.g., phase composition, crystallinity, interfacial strain, and, at the same time, the concentration of aqueous species in the solution, from which, in some cases, even the local pH could be determined. Moreover, due to the isotope shift of vibrational modes FC CHRS additionally facilitates to follow the exchange of oxygen and hydrogen isotopes among solids and aqueous molecular species, if isotopically enriched precursors are used. Intriguing proofs of concepts for such in situ FC CHRS studies are given by the results of first experiments which were performed to study the aqueous corrosion of borosilicate glass at 70°C and the replacement of celestine by strontianite in a sodium carbonate solution at 21°C. The experiments were carried out in home-made fluid cells with a sample arrangement that allows measuring the solution and the formation of the reaction product with a micrometer resolution. The results of these experiments are very encouraging. They revealed, in both cases, unpredicted, short-term changes in the mechanism and the overall reaction kinetics. They further demonstrate that dynamic phenomena such as re-equilibration reactions and potentially the diffusion of molecular water through the product layer can be studied and quantified when using 18-O and 2-H as tracers. FC CHRS thus potentially opens up the possibility to gain deeper insights into solid-water reactions. The main objectives of the here proposed project are (A) to develop/advance and eventually establish the experimental method, (B) to in situ study the mesoscopic mechanism(s), and (C) to identify and quantify the rate-limiting steps controlling the corrosion of borosilicate glasses and the replacement of celestine by strontianite at elevated temperatures. The planned experiments will deliver unique insights into the mechanism(s) and kinetics of replacement reactions in aqueous solutions, providing the basis to develop and test numerical models.

• DFG Programme: Research Grants