Novel use of borosilicates in geothermobarometry

Although many articles about minerals from the tourmaline and dumortierite supergroups have been published, detailed investigations of the relation between crystal chemistry and pressure-temperature conditions are still missing. A systematic investigation of natural borosilicates from different geological environments, which are petrologically well characterized, as well as synthetic borosilicates, which are synthesized under defined pressure-temperature conditions, can lead to important relationships. It is hypothesized that the most important borosilicates are suitable for geothermobarometry. The distribution of the elements Fe, Mg and Al at the different sites in the crystal structure of tourmaline could be an important finding to successful calibrate such a tourmaline geothermometer. It is also possible that in Mg-bearing tourmalines at higher pressures such an intersite disorder positively correlates to B at the Si position. Further it will be checked, if the slope of the positive correlations between the average charge of the cations at the Na-position (X site) and the F content in tourmalines in closed systems is related to the temperature during the crystallization of such a tourmaline. It will also be checked, if in natural dumortierite, the Mg content is positively correlated to the pressure conditions and to the OH content. Borosilicates will be synthesized at different pressure-temperature conditions. Crystal structures of synthetic and natural tourmalines and dumortierites from petrological well-defined rocks will be determined by the single-crystal diffraction method. The complete chemical composition of these borosilicates will be determined. Finally, relationships between crystal chemistry and pressure-temperature conditions are developed. Although many scientists see great potential for the use of these borosilicates in geothermobarometry, many questions have not yet been solved. Especially the orderdisorder of Mg, Fe and Al in octahedral sites as well as B and Al at the tetrahedral sites can lead to interesting correlations to pressure-temperature conditions. Completely new studies are being conducted to establish correlations, which seem to be temperature dependent, e.g., between the average charge at the X site and the F content on samples of different origins. Preliminary investigations on high-pressure tourmalines show that, with increasing disorder of Mg and Al between the different crystallographic sites, the amount of B at the Si position increases, while an increasing symmetry deviation from the trigonal crystal system occurs. This is a completely new observation. This strategic research includes many new ideas and will be particulary helpful for the use of borosilicates in geothermobarometry. This project will benefit from many highly qualified international collaborators with significant skills and experience.