In situ high-temperature behaviour of fluor-elbaite: breakdown conditions and products

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.

In situ high-temperature behaviour and breakdown conditions of uvite at room pressure

“…The observed assemblage of phases is significantly different with respect to that resulting from the thermal breakdown of both Fe-rich fluor-elbaite (Celata et al 2021) and Mn-bearing fluor-elbaite (Ballirano et al 2022). In particular, the mullite-like phase reported as the prevailing crystalline product of Fe-rich fluor-elbaite and Mn-bearing fluorelbaite breakdown is replaced by indialite.…”

“…At the end of the heating run, the capillary was cooled back at room temperature (RT) within the chamber with a 10 °C/min rate. The powder was removed from the capillary, re-homogenized and loaded in a new borosilicate-glass capillary following the same procedure, aimed at reducing the possible effect of textured recrystallization at the walls of the capillary, adopted by Celata et al (2021) and Ballirano et al (2022). Data collection was performed keeping the sample outside the chamber using the same angular range, step-scan size, and counting time of the HT measurements (see Table 1).…”

“…Owing to the limited amount of "boron mullite" in mixture, it was not possible to derive reliable unit-cell parameters to characterize in detail its chemistry as it was done for the Al 2 O 3 -B 2 O 3 -SiO 2 (ABS) phases arising from the breakdown of fluor-elbaite samples (Celata et al 2021;Ballirano et al 2022).…”

In-situ high-temperature behaviour and breakdown conditions of uvite at room pressure

“…Table 3 lists the relevant parameters of the tting procedure. Data indicate that the c-parameter is softer than the a-parameter against T. It is worth noting that uvite expands at a rate slightly greater than that of uro-elbaite (Celata et al 2021).…”

“…The observed assemblage of phases is signi cantly different with respect to that resulting from the thermal breakdown of both uor-elbaite (Celata et al 2021) and Mn-bearing uor-elbaite (Ballirano et al 2022). In particular, the mullite-like phase reported as the prevailing crystalline product of uor-elbaite and Mn-bearing uor-elbaite breakdown is replaced by indialite.…”