A new method for the determination of novel tourmalines

The aim of this project is to draw conclusions about the chemical composition and the elemental distribution in tourmaline using structural data (from single crystal X-ray data). This seems to be important because of the complex crystal chemistry of tourmaline, which can also incorporate different amounts of light elements like H, Li, and B. A complete chemical characterisation of a tourmaline needs a high expenditure of time (e.g., analyses with electron microprobe, secondary ion mass spectroscopy, Mössbauer spectroscopy). Furthermore, because of the elemental distribution in the structure, many times a crystal structure determination is necessary. An indirect chemical characterisation of a tourmaline by using just structural data would be very fast. In only a few hours the X-ray data of a tourmaline crystal can be collected either from a crystal fragment or from a thin section, and hence the tourmaline can be classified immediately by using the structural data as input for a computer program. For this project tourmalines of complex solid solutions will be characterised structurally (by single-crystal X-ray methods) and chemically (including Mössbauer spectroscopy as well as analyses of the light elements H, Li and B) to work out correlations between the chemistry and the structural data. Thus, in the future, tourmaline samples from different geological environments could be characterised very quickly and assigned to a combination of different end members. This seems to be important, because tourmaline is a significant petrologic indicator of different geologic environments. Because the elemental distribution in the structure of tourmaline reflects both the PT conditions and the fluid history of rocks in which it develops, conclusions about the conditions of formation of the different lithologies can be drawn.

Investigations were done on tourmaline, a common mineral group of strongly varying composition, which belongs to the borosilicates. Several correlations between the crystal chemistry and the structural features and also other important factors like temperature and pressure, were found. Investigations were done on natural and synthetic Al- rich tourmalines. Towards lower temperatures higher amounts of B, substituting Si, could be verified. Above a pressure of 1000-1500 MPa the highest observed [4] B content does not change significantly any more at a given temperature. Further investigations were done on Li-, Fe2+- and Mn 2+-rich tourmalines. In a pegmatitic system where essentially no Fe, Mn, Ti and Mg are available, the Li content is an important factor, which seems to control the amount of Si in tourmaline. Once the Li content is lower, Al cations must occupy this atomic position. For a charge-balanced formula, other cation sites must therefore have lower bulk charges. This can be achieved by increasing vacancies at different positions and increasing amounts of trivalent cations at the Si site. Another investigation on Mg-rich tourmalines from the pegmatite-marble contact from the Austroalpine basement units, was done. Tourmalines from the contact zone of Permian pegmatitic rocks to micaschists and marbles from the Austroalpine basement units in Styria, Austria, were characterized. All these Mg-rich tourmalines have small but significant Li contents and can be characterized as Fe-bearing dravite. There is a positive correlation between MgO of the tourmalines and the surrounding micaschists. We conclude that the tourmalines may have crystallized in the contact between a pegmatitic melt formed by local anatexis and contact processes during the emplacement of the Permian pegmatites with carbonatic and metapelitic host rocks. Another study on tourmalines from rocks within ultrahigh-pressure (UHP) metamorphic localities was also published. These tourmalines have been subjected to a structurally and chemically detailed analysis to test for any systematic behaviour related to temperature and pressure. There is no structural evidence for significant substitution of Si by Al or B in UHP tourmaline, even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-temperature-low-pressure tourmaline, which typically has significant amounts of [4] Al. There is an excellent positive correlation between total Al and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a pronounced negative correlation between F content and the temperature conditions of UHP tourmaline formation. Chemical, structural and spectroscopic data were obtained on tourmalines of the elbaite-schorl solid solution from San Diego County, California. Lithium abundance increases from core to rim of a large tourmaline crystal, whereas Mn 2+ and F increase, reach a maximum, and then decrease. There is an excellent inverse correlation between the lattice parameter a and the Li content. Positive correlations between [6] Al and [4] B, and between (Mn2+ + Fe2+) and [4] Al were found in these tourmalines. Intensive work was also done on the nomenclature of the tourmaline supergroup minerals. A new F- and Fe2+-rich tourmaline was recently approved as the new mineral fluor-schorl.