Compressibility of synthetic tourmaline of near end-member composition up to 60 GPa
Abstract
As the primary boron host in rocks of various bulk compositions and formation conditions including subduction-related rocks, tourmaline plays a pivotal role in the boron cycle. Tourmaline is a supergroup mineral (>33 end-member species) with the general structural formula XY3Z6T6O18(BO3)3V3W. As a common accessory phase with highly variable chemistry and extensive stability, tourmaline is known to provide a petrological and geochemical record of fluid-rock interaction in its host. However, its fundamental thermodynamic properties remain largely unknown, hindering tourmaline's inclusion in petrological models. Knowledge of its non-ambient elastic properties is scarce and largely limited to studies on natural samples to <18 GPa. Reported bulk moduli are variable and it remains unclear if this reflects variation in sample composition or data-collection method.
Synthetic crystals allow the properties of end-members to be investigated directly, which can then be combined to reflect the properties of their solid solution in natural samples. We present high-pressure single-crystal X-ray diffraction data collected in a standard symmetric diamond anvil cell up to 60 GPa using synthetic tourmaline of near end-member composition. A pressure-volume equation of state was constrained for tourmaline representing each of the following end-members: dravite (Na-Mg-Al tourmaline), oxy-uvite (Ca-Mg-Al tourmaline), magnesio-foitite (X-vacant-Mg-Al tourmaline), and olenite (Na-Al tourmaline) Results show tourmaline's compressibility is sensitive to composition. In particular, olenite's high Al content is correlated with an increased bulk modulus. X-site vacancies are correlated with a minor increase in compressibility in the a axis direction. In addition, tourmaline's compressibility is highly anisotropic. Each investigated crystal is 3 times more compressible along the crystallographic c axis than along the a axis at ambient conditions. With increasing pressure, anisotropy in the axial compressibility decreases. These results reveal the importance of the octahedral-site occupancy on tourmaline's elastic properties. Moreover, this study emphasizes the value of synthetic crystals in characterizing compositional effects in this complex crystal system.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V33C0240B
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGYDE: 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 3640 Igneous petrology;
- MINERALOGY AND PETROLOGYDE: 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY