Author: Ferrow, Embaie

Source: European Journal of Mineralogy, Volume 21, Number 1, February 2009 , pp. 241-250(10)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

Abstract:

The tourmaline group of minerals is indeed an enigma. Experimental data from optical spectroscopy, electron microscopy, and Mössbauer spectroscopy reveal a host of physical properties that lack a common structural clarification. For example, tourmaline samples change colour when irradiated with X-ray and γ-ray radiations - some reverting back when heated in air; exhibit simultaneous oxidation and reduction on annealing in an atmosphere of H₂; display different plane group symmetries under TEM; possess the most complicated Mössbauer spectra of all Fe-bearing silicates. In this study, four Brazilian samples were chosen for detailed study by Mössbauer spectroscopy to find out a common structural factor to the physical anomalies reported in the literature. It was found out that the tourmaline group of minerals contain multi-valence elements that are involved in electron exchange between the edge-sharing asymmetric Y and Z crystallographic sites. It is conceivable that the host of physical properties recorded in the literature could be due to the inherent structural misfit between the Y and Z sites and the mechanisms adopted to reduce the strain associated along the shared edges. The complexity of non-integral oxidation states possible - due to electron sharing among the different multi-valence elements present in the structure - further enhances the diverse physical properties observed. Moreover, on heating in air, no net oxidation or reduction takes place in the tourmaline group of minerals over a temperature range as long as there are electron donors and acceptors left in the structure, serving simultaneously as potential single-phase buffers and geo-thermometers.

Keywords: TOURMALINE; MÖSSBAUER; HYBRID IONS; BUFFER; GEO-THERMOMETER

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2009/0021-1863

Publication date: 2009-02-01

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