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 <Y-O>, <Z-O> and <T-O> 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).

No additional funding sources recorded.