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

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 degrees 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 degrees C (1123 K) and 900 degrees 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) angstrom, b = 7.6749(2) angstrom, c = 2.8385(1) angstrom, V = 164.22(1) angstrom(3) (space group Pbam) consistent with an approximate Al8.5B1.5Si2O19 composition.

Automated summary

The thermal behavior of a fluor-elbaite from Minas Gerais, Brazil was studied using in situ high-temperature X-ray powder diffraction. Two main internal reactions were identified: a Fe oxidation process and (OH)(-) deprotonation, followed by a Fe-Al exchange between octahedrally-coordinated sites. The breakdown reaction of the fluor-elbaite occurs between 850 and 900 degrees Celsius, producing B-bearing mullite, hematite, spinel, and B-poor (Na, Li, H2O)-bearing glass as products. Boromullite was not observed in the final heating products.