Correlations between tourmaline crystal chemistry and PT conditions during crystal growth

The project, which is planned for three years, aims at the structural characterisation and classification of natural and synthetic tourmalines to enlarge our knowledge about effects of inter-site partitioning and interactions. It should be mentioned that although a lot of articles about tourmaline were published in the past, a detailed investigation about the relation between crystal chemistry and pressure-temperature (PT) conditions is still missing. This strategic research will, firstly, provide a very important knowledge and data base for future studies in the innovative fields of applied petrology and mineralogy. A systematic study of different tourmalines from different geologic environments, which are petrologically well characterised, can lead to important correlations especially of Mg-Al disorder between the Y site and the Z site ([6]-coordinated sites in the tourmaline structure), the T-site occupancy, bond lengths, bond angles, occupancies, bond-angle distortions, bond-length distortions, the chemistry and between PT conditions. More recent investigations indicate that such relationships are relatively complex. The estimated OH and F contents are also of interest, because tourmaline is an important petrogenetic indicator for fluid-rock interactions. It has to be checked, if the positive correlation between the average charge at the X site and the F content is dependent on the temperature during tourmaline crystallisation as well as on the tourmaline solid solution which is influenced by the bulk composition of the surrounding rock. More detailed investigations (in low grade to high grade environments) have to be done to further characterise the correlation between distances and the temperature conditions during tourmaline crystallisation. This would be very important for the understanding of the Mg-Al disorder between the Y and the Z sites. It has to be checked, if there is a general correlation between [4]Al in tourmaline and the temperature during tourmaline crystallisation and how strong it depends to the bulk composition of the rock. It is further of special interest how the occupations of the X, Y, Z, T and W site (Fluorine occupies this site) are related to each other, and how the occupation of these sites is related to the temperature during tourmaline crystallisation. Helpful are maybe also investigations of tourmalines (with different compositions) which are synthesised at different temperatures (also planned for this project) as well as heating experiments on natural tourmalines (of different solid solutions) from different environments. Finally, these results may promote the general development of petrological (tourmaline geothermobarometry) and mineralogical aspects, also helpful for synthesising relatively large (coloured) gem-quality tourmalines for different technical applications and also for the use as gemstones.

Tourmaline is a very important mineral group because of its relevance in the geosciences, for its technical applications and its use as gemstone. Tourmalines are complex aluminium-borosilicates with strongly varying compositions because of their structural flexibility. The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals, with the general formula X Y3 Z6 [T6O18] (BO3)3 V3 W. Because of this chemical flexibility tourmaline crystals are in most cases not colorless, but can show a lot of different colors. The investigations focused on correlations between tourmaline crystal chemistry, structural data and formation conditions to find useful correlations which could also be useful for the geosciences in general. A pronounced negative correlation between temperature during crystal growth (at a constant pressure) and B at the Si (T) site in synthetic Al-rich tourmalines was found. Detailed investigations were done on Fe- and Mn-rich tourmalines, which showed that Fe2+ and Mn2+ do not occupy the Z site. Heat treatment of these tourmalines at 700 C for up to 1.5 days oxidised Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange of Al at the Z site, which was observed for the first time. Structural, chemical and spectroscopic data were used to characterize tourmalines from different granitic pegmatites of the Moldanubian nappes, Lower Austria. A positive correlation between the average T-O and Z-O bond lengths in tourmalines where the Z site is only occupied by Al is useful to correct the average Z-O bond length for the inductive effect of the varying average T-O bond length. This is important for producing accurate assignments for the different atomic sites in tourmaline. On the basis of determined ages, the pegmatites crystallized during the Variscan metamorphic event. These ages are in the range of the earliest intrusions of the South Bohemian pluton. The pegmatites may have evolved as granitic pegmatitic melts during decompression from the surrounding country rocks in the frame of exhumation of the Moldanubian nappes after the peak of the Variscan metamorphism. Darrellhenryite, a new member of the tourmaline supergroup, was firstly described as Al-rich, Li-bearing and OH-deficient tourmaline. It occurs in a complex pegmatite in southern Bohemia, Moldanubian Zone, Czech Republic. Mn- and Cu-bearing tourmalines from Brazil and Mozambique were characterised chemically and by single-crystal structure refinement. The samples at each locality show a strong negative correlation between the X-site vacancies and the Mn + Fe content. The Mn content in these tourmalines depends, besides the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between Mn and Cu it can be argued that the Cu content in tourmaline depends, besides the availability of Cu, on stereochemical constraints and not on the formation temperature.