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.