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In Situ High-Temperature Behaviour of Fluor-Elbaite: Breakdown Conditions

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In Situ High-Temperature Behaviour of Fluor-Elbaite: Breakdown Conditions Physics and Chemistry of Minerals (2021) 48:24 https://doi.org/10.1007/s00269-021-01147-5 ORIGINAL PAPER In situ high‑temperature behaviour of fuor‑elbaite: breakdown conditions and products Beatrice Celata1 · Paolo Ballirano1 · Giovanni B. Andreozzi1 · Ferdinando Bosi1 Received: 31 March 2021 / Accepted: 19 May 2021 / Published online: 8 June 2021 © The Author(s) 2021 Abstract The thermal behaviour of a fuor-elbaite from Minas Gerais (Brazil) was investigated at room pressure through in situ high- temperature X-ray powder difraction (HT-XRPD), until the breakdown conditions were reached. The variations of fuor- elbaite structural parameters (unit-cell parameters and mean bond distances) were monitored together with site occupancies, and two main internal reactions were identifed: the thermally-induced Fe oxidation process counterbalanced by (OH)– depro- tonation, which starts at 500 °C (773 K), followed by a partial intracrystalline Fe–Al exchange between the octahedrally- coordinated Y and Z sites. The fuor-elbaite breakdown reaction occurs between 850 °C (1123 K) and 900 °C (1173 K). The breakdown products were identifed 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 fnal 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. Keywords Fluor-elbaite · HT-XRPD · Thermal expansion · Iron oxidation · Deprotonation · Intracrystalline cations exchange · Structural breakdown Introduction crystallographic sites (italicized letters) and the letters V and W represent groups of anions accommodated at the [3]-coor- Tourmaline is one of the most fascinating and colourful dinated O3 and O1 crystallographic sites, respectively. accessory mineral occurring in a variety of geological envi- Tourmaline-supergroup minerals are currently classifed into ronments, from diagenetic stages to granulite facies grade three groups, vacant, alkali and calcic, based on the X-site (e.g., Henry and Dutrow 1996; Dutrow and Henry 2011; occupancy (Henry et al. 2011). A further level of classifca- Bosi et al. 2018a, 2019a; Andreozzi et al. 2020). tion into subgroups is based on the charge arrangements at Tourmaline is a cyclosilicate rich in B with a very com- the Y and Z sites. Tourmalines are also distinguished by the plex composition represented by the general chemical for- dominant anion at the W position of the general formula into + + 2+ mula: XY3Z6T6O18(BO3)3V3W, where X = Na , K , Ca , hydroxy-, fuor- and oxy-species. □ (= vacancy); Y = Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+, Tourmaline gained more and more interest along the Mn2+, Li+; Z = Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+; T = Si4+, years surely because of its remarkable power to carry a lot Al3+, B3+; B = ­B3+; V = (OH)−, O2−; W = (OH)−, F−, O2−. of information about its genetic conditions (e.g., Federico Note that the non-italicized letters X, Y, Z, T and B rep- et al. 1998; Dutrow and Henry 2011). However, tourma- resent groups of cations at the [9]X, [6]Y, [6]Z, [4]T and [3]B line relevance stands even more in its role of boron and water carrier from the crust deep down the mantle and the * Beatrice Celata implications it may have (Henry and Dutrow 1996; Ota [email protected] et al. 2008a, b; Shimizu and Ogasawara 2013; Lussier et al. * Paolo Ballirano 2016). In fact, the boron and water released because of tour- [email protected] maline breakdown reduce both the solidus temperature of the hosting rock and the viscosity of any associated melt 1 Dipartimento di Scienze della Terra, Sapienza Università di (Pichavant 1981; Dingwell et al. 1992). Remarkably, how Roma, Piazzale A. Moro 5, 00185 Rome, Italy Vol.:(0123456789)1 3 24 Page 2 of 8 Physics and Chemistry of Minerals (2021) 48:24 Table 1 Miscellaneous data of the breakdown conditions are reached in terms of structural 2θ range (°) 7–145 variations has never been investigated so far, as well as it the data collection and Rietveld refnements of the fuor-elbaite 2θ step-size (°) 0.021798 remains unclear when the deprotonation process exactly studied Counting time (s) 3 starts, i.e., if "water" is actually released throughout the Tmax (°C) 850 breakdown process or way before the structural collapse as T steps (°C) 50 suggested, for example, by the studies of Filip et al. (2012) Rp (%) 1.772–2.275 and Bosi et al. (2018b). Rwp (%) 2.392–3.312 The present work aims at investigating the thermal behav- RBragg (%) 0.907–1.303 iour of the fuor-elbaite, ideally Na(Li1.5Al1.5)Al6(Si6O18) DWd 0.801–1.363 (BO3)3(OH)3F (Bosi et al. 2013) at room pressure. A Fe- Χ2 1.767–2.509 bearing deep green fuor-elbaite sample from the Cruzeiro pegmatite (Minas Gerais, Brazil), previously fully character- Defnition of the statistical indi- ized by Bosi et al. (2019b) with the formula: cators, from Young (1993) X(Na ◻ Ca K ) Y Al Fe2+ Li Mn2+ Zn Ti 0.79 0.18 0.02 0.01 Σ1.00 0.95 0.94 0.86 0.18 0.11 0.01 Σ3.05 Z T V W Al6 Si5.99Al0.01 B2.98O27 (OH)3 (OH)0.43F0.58 Σ6.00 Σ1.01 was studied by in situ high-temperature X-ray powder dif- Data evaluation was performed by the Rietveld method fraction (HT-XRPD) up to the structural breakdown. using Topas 6 (Bruker 2016). The Fundamental Parameters Approach (FPA: Cheary and Coelho 1992) was used to describe the peak shape. The equation of Sabine et al. (1998) for a cylin- Experimental drical sample was applied for absorption correction using the approach of Ballirano and Maras (2006) for handling the cor- The fuor-elbaite crystal fragment was gently grinded in relation existing between displacement parameters and absorp- ethanol, in an agate mortar; the powder was then loaded in tion. In particular, isotropic displacement parameters were constrained as follow: BY = BZ = BB = BT ; a 0.7 mm diameter SiO2-glass capillary kept open at one side. The capillary was fxed to a hollow corundum tube BO2 = ­BO3 = ­BO4 = ­BO5 = ­BO6 = ­BO7 = ­BO8. Preferred orientation using a HT cement and mounted and aligned on a goniome- efects were corrected using spherical harmonics (8th-order, ter head. The capillary was inserted into the heating chamber nine refnable parameters) by selecting the number of appropri- for capillaries, developed by MRI and Bruker AXS, that is ate terms following the procedure described by Ballirano placed along the beam path of the difractometer. Character- (2003). As expected for data collected in transmission-mode on istics and thermal calibration procedure of the chamber are capillaries, the coefcients refned to small values. Starting reported in Ballirano and Melis (2007). structural data were those of Bosi et al. (2019b) and each T In situ HT-XRPD data were measured on a Bruker AXS refned structure at a given non-ambient was used as input for T D8 Advance that operates in θ/θ geometry in transmission the subsequent . EoSFit7-GUI (Gonzalez-Platas et al. 2016) mode. The instrument is ftted with focussing multilayer was used to analyse the dependence of the unit-cell parameters T graded (Göbel) mirrors placed along the incident beam and from employing the equation of Berman (1988) for ftting the Soller slits on both the incident (2.3° opening angle) and data. This equation has the advantage to permit accommodation of non-linear thermal expansion. It is expressed as difracted (radial) beams. The data were collected using a 1 2 X = X0 1 + a0 T − T + a1 T − T X V a position sensitive detector (PSD) VÅntec-1 operating at an T ref 2 ref with = , , opening angle of 6° 2θ. Details of the data collection are c. Miscellaneous information regarding the refnements is listed reported in Table 1. in Table 1 and a representative example of Rietveld plots is At the end of the heating run, the powder was cooled back shown in Fig. 1. CIF fles of the fuor-elbaite structure refned at room temperature (RT) within the chamber, removed from at the various T are given in Online Resource. the capillary, re-homogenised and charged in a new borosili- cate–glass capillary. This procedure was followed to reduce the possible efect of textured recrystallization at the walls of Results and discussion the capillary. As a side efect, re-homogenisation involved also powder lying at the coldest extremity of the capillary where T, Breakdown products of F‑elbaite owing to thermal gradients, was signifcantly smaller than that reached in the analysed part of the sample. A measurement of The frst evidence of fuor-elbaite structural breakdown was this sample was performed outside the chamber. observed at 850 °C owing to the occurrence of very weak 1 3 Physics and Chemistry of Minerals (2021) 48:24 Page 3 of 8 24 Fig. 1 Representative example of the Rietveld plots of the difraction pattern collected at 423 K. Blue: experimental; red: calculated; grey: difer- ence; vertical bars: position of calculated Bragg refections of the fuor-elbaite studied difraction refections assigned to a mullite-like phase. The phase plus minor hematite and traces of spinel as breakdown breakdown was completed at the T of 900 °C. The quantita- products. Furthermore, the increased intensity of the broad tive phase analysis (QPA) of the sample cooled down at RT band centred at ca. 22° 2θ, in addition to the contribution (Fig.
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