Detailed investigations of metamorphic and magmatic tourmaline as a geothermometer

The project, planned for three years duration, aims at the structural characterisation and classification of metamorphic and magmatic tourmalines to enhance our knowledge about effects of inter-site partitioning and interactions. Although many articles about tourmaline have been published in the past, a detailed investigation of the relation between crystal chemistry and pressure-temperature (PT) conditions is still missing. A systematic study of tourmalines from different geologic environments, which are petrologically well-characterised, can lead to information on important correlations, especially between the ordering of Mg and Fe3+ into the Y site and the Z site, the amount of F in relation to the X-site charge (Na, Ca), the amount of [4]Al, and structural features such as bond lengths, bond angles, refined occupancies, bond-angle and bond-length distortions in different PT conditions. Preliminary investigations show that for the investigated Mg- and Fe-bearing tourmaline samples that crystallised at varying PT conditions (up to 10 kbar/100-750 C), the proportion of Mg/YMg as well as of ZFe/YFe tends to decrease at higher temperatures. To get more accurate Z correlations, it is absolutely necessary to extend the detailed investigations on metamorphic and magmatic tourmalines, using samples which crystallised in well-characterised host rocks that experienced a wide range of PT conditions. For the first time it is possible to assign the Y- and Z-site occupants in tourmaline precisely using a new method, which was developed by the applicant. This is a requirement to further examine possible correlations between the Y- and Z-site occupation and PT conditions. This new method considers inductive effects between , and distances. Such effects are known to exist but they were never considered mathematically by doing site assignment in the past. It must be determined if there is a general correlation between [4]Al in tourmaline and the temperature during tourmaline crystallisation and how strongly it depends on the bulk composition of the rock. Hence the bulk composition will also be estimated and, if necessary, the PT conditions of the (equilibrated) mineral paragenesis (including tourmaline) will be estimated using conventional geothermobarometry. It is further of particular interest how the occupants of the X, Y, Z, T and W site (F occupies this site) are related to each other, and how the occupation of these sites is related to the temperature during tourmaline crystallisation. Such correlations might be helpful to learn more about the temperatures during tourmaline crystallisation, including Li-rich tourmaline. This strategic research will provide a very important knowledge and data base for future studies in the innovative fields of applied petrology (tourmaline geothermobarometry) and mineralogy, and also be helpful for synthesising relatively large gem-quality tourmalines for different purposes (technical applications and also for the use as gemstones).

The investigations focused on correlations between crystal chemistry and structural data of tourmaline, considering also petrological and geological data to use tourmaline in future as geothermometer. This would be of special interest for the whole Geosciences, because tourmaline is quite common in almost all rocks. In low-temperature tourmalines (magnesio-foitite and foitite) prefers Al strongly the Z site, whose octahedra form a network of chains parallel to the c direction. Therefore, the Al enrichment, which is common in acicular to fibrous tourmalines, can be an important factor promoting preferential growth in the c direction. A synthetic Cu-rich tourmaline crystal consists of three different zones. While [4]Al decreases, [4]B increases, which could be explained due to a decreasing temperature during tourmaline crystallization. Two of the six (Cu,Al)-O distances are significantly enlarged, while the other distances are significantly smaller. The proportion of the average of the two enlarged distances to the average of the other distances in the Cu-richest zone verified for the first time the Jahn-Teller effect in a tourmaline. The use of compositionally simple tourmalines (synthetic K-dravite, dravite, oxy-uvite, magnesio-foitite) facilitates the determination of site occupancy in the structure refinement and band assignment in the Raman spectra. These tourmalines have significant MgAl disorder between their octahedral sites. The MgAl ratio of these sites was determined from the relative integrated peaks of the Raman bands in the OH stretching vibrational range, producing values in good agreement with the crystal structure refinement data. Extensive investigations were done on Fe2+-rich tourmaline with varying OH/F contents. Fluor-schorl, a new mineral species of the tourmaline supergroup could be described from alluvial tin deposits from the Saxony Ore Mountains, Germany, and from pegmatites in Trentino, South Tyrol, Italy. It was formed as pneumatolytic phase and in high-temperature hydrothermal veins in granitic pegmatites and is related to end-member schorl by the substution F (OH). It was found that a complete fluor-schorl schorl solid-solution series exists. A strong positive correlation exists between the refined F content and the YW distance, which may be used to quickly estimate the F content in unknown tourmalines. Olenitic tourmaline with high amounts of tetrahedral B (up to ~2.5 atoms per formula unit) has been synthesized at 4 GPa, 700 C. The major- and trace-element composition of Cu-bearing tourmaline was investigated from three different localities. Based on a comparison of unheated reddish tourmalines with blue ones, it was suggested that blue Cu-bearing tourmalines with CuO/MnO <0.6 may have been heat-treated to reduce the contribution of the reddish component of Mn3+. Bosiite, a new Fe3+- and Mg-rich tourmaline was described as a hydrothermal phase from the Darasun gold deposit, Russia. It is related to end-member povondraite by the substitution of Al Fe3+ at the Z site. The chemical composition of a synthetic rossmanitic tourmaline, which was synthesized at 4 GPa and 700C, showed that a significant amount of Li must occupy the X site for charge balance requirements. Reliable assignment of the OH-stretching vibrations in a polarized single-crystal Raman spectrum and a single-crystal X-ray structure refinement, confirm the incorporation of Li at the X site for the first time. These findings are important for the understanding of one of the most common mineral groups, the tourmaline group.