Composition, temperature, and pressure dependence of phonon modes in complex hydrous silicates

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Amphiboles (AB2C5T8O22W2, C = M12M22M3) and tourmalines (XY3Z6T6O18(BO3)3V3W) are two mineral supergroups with a rather complex chemical composition, having several crystallographic sites that can be occupied by various chemical elements. Crystal-chemistry studies of amphiboles and tourmalines have important implications in Earth sciences, in particular when using petrologic models based on major and minor elements to analyze the type and petrogenesis of host rocks. Such studies are also vital to better understand the thermodynamic behaviour of minerals at various temperatures and pressures as well as the compositional dependencies of mineral properties. Hence, they have implications also in materials science and environmental science as well as in gemmology and studies of cultural-heritage artefacts. Phonons are the microscopic origin of elastic and thermal properties and therefore, it is of great significance to study the influence of each of the three key thermodynamic variables: composition x, temperature T, and pressure p, on the atomic dynamics. A comprehensive analysis of the relation between (x, T, p) and phonons in amphiboles and tourmalines was the objective of the previous project MI 1127/7-1 and the satellite project SCHL 549/6-1. Thus, the major goal of this continuation project is to further extend the fundamental understanding of the relation between phonon density of states and crystal chemistry in complex hydrous silicates by studying more mineral species as well as to complete the study of thermal and elastic behaviour of amphiboles. The three main aims are: (i) analysis of the chemistry-phonon interplay in the Cr-V-Al ternary system of alkali tourmalines; (ii) studies of the influence of tetrahedrally coordinated Al as well as of octahedrally coordinated Al and Fe3+ on the OH-stretching and framework vibrations of amphiboles, and (iii) studies of T and p evolution of phonon modes in various amphibole species. The one-mode/two-mode approach will be further used to study the tourmaline series: oxy-dravite - oxy-vanadium-dravite - oxy-chromium-dravite. The results will be also compared to those for the corresponding trivalent-iron counterparts, to better understand the effect of Y- and Z-site d elements on the atomic dynamics of tourmalines. The studies of composition-phonon relations for amphiboles will be extended to framework vibrations as well as to TAl-containing amphiboles species. Special attention will be also given to ferri amphiboles. The chemical renormalization of T- and p-induced phase transitions in amphiboles is going to be studied via the trends of phonon wavenumbers and widths. Oxidation processes in Fe-containing amphiboles are going also to be studied via in situ Raman spectroscopy up to 1400 K and the results will be compared with those achieved for Fe-bearing tourmalines.