Abstract
The thermal behaviour of a fluor-elbaite from Minas Gerais (Brazil) was investigated at room pressure through in situ high-temperature X-ray powder diffraction (HT-XRPD), until the breakdown conditions were reached. The variations of fluor-elbaite structural parameters (unit-cell parameters and mean bond distances) were monitored together with site occupancies, and two main internal reactions were identified: the thermally-induced Fe oxidation process counterbalanced by (OH)– deprotonation, which starts at 500 °C (773 K), followed by a partial intracrystalline Fe–Al exchange between the octahedrally-coordinated Y and Z sites. The fluor-elbaite breakdown reaction occurs between 850 °C (1123 K) and 900 °C (1173 K). The breakdown products were identified at room temperature by XRPD and the breakdown reaction can be described by the following reaction: tourmaline → B-bearing mullite + hematite + spinel + B-poor (Na, Li, H2O)-bearing glass. Boromullite itself was not observed in the final heating products, and the B-bearing mullite from the breakdown reaction exhibited unit-cell parameters a = 7.5382(2) Å, b = 7.6749(2) Å, c = 2.8385(1) Å, V = 164.22(1) Å3 (space group Pbam) consistent with an approximate Al8.5B1.5Si2O19 composition.
Highlights
Tourmaline is one of the most fascinating and colourful accessory mineral occurring in a variety of geological environments, from diagenetic stages to granulite facies grade (e.g., Henry and Dutrow 1996; Dutrow and Henry 2011; Bosi et al 2018a, 2019a; Andreozzi et al 2020).Tourmaline is a cyclosilicate rich in B with a very complex composition represented by the general chemical formula: XY3Z6T6O18(BO3)3V3W, where X = Na+, K+, Ca2+, □ (= vacancy); Y = Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+, Mn2+, Li+; Z = Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+; T = Si4+, Al3+, B3+; B = B3+; V = (OH)−, O2−; W = (OH)−, F−, O2−
The first evidence of fluor-elbaite structural breakdown was observed at 850 °C owing to the occurrence of very weak
The modifications observed by Bosi et al (2019b) on a single-crystal fragment of the same fluor-elbaite studied in this work, heated in air at the T of 800 °C and studied at room temperature (RT) by single crystal X-ray diffraction, confirmed that Na was not released after heating, as reported in their empirical formula, X(Na0.80◻0.16Ca0.03K0.01)Σ1.00 Y Al1.13Fe30+.62Fe20+.07Li0.87Mn0.18Zn0.12Ti0.01 Σ3.00
Summary
Tourmaline is one of the most fascinating and colourful accessory mineral occurring in a variety of geological environments, from diagenetic stages to granulite facies grade (e.g., Henry and Dutrow 1996; Dutrow and Henry 2011; Bosi et al 2018a, 2019a; Andreozzi et al 2020). Tourmaline gained more and more interest along the years surely because of its remarkable power to carry a lot of information about its genetic conditions (e.g., Federico et al 1998; Dutrow and Henry 2011). Tourmaline relevance stands even more in its role of boron and water carrier from the crust deep down the mantle and the implications it may have (Henry and Dutrow 1996; Ota et al 2008a, b; Shimizu and Ogasawara 2013; Lussier et al 2016). The boron and water released because of tourmaline breakdown reduce both the solidus temperature of the hosting rock and the viscosity of any associated melt (Pichavant 1981; Dingwell et al 1992).
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