HT breakdown of Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar

Abstract

The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition X (Na 0.41 □ 0.35 Ca 0.24 ) Σ1.00 Y (Al 1.81 Li 1.00 Fe 3+ 0.04 Mn 3+ 0.02 Mn 2+ 0.12 Ti 0.004 ) Σ3.00 Z Al 6 [ T (Si 5.60 B 0.40 ) Σ6.00 O 18 ](BO 3 ) 3 (OH) 3 W[(OH) 0.50 F 0.13 O 0.37 ] Σ1.00 was investigated using both in situ High-Temperature X-Ray powder diffraction (HT-pXRD) and ex situ X-Ray single-crystal diffraction (SC-XRD) on two single crystals previously heated in the air up to 750 and 850 °C. The first occurrence of mullite diffraction peaks allowed us to constrain the breakdown temperature of Mnbearing elbaite at ambient pressure, at 825 °C. The breakdown products from the HT-pXRD experiments were cooled down to ambient temperature and identified via pXRD, represented by B-mullite and γ-LiAlSi 2 O 6 . A thermally induced oxidation of Mn 2+ to Mn 3+ was observed with both in-situ and ex-situ techniques; it started at 470 °C and is assumed to be counterbalanced by deprotonation, according to the equation: Mn 2+ + (OH) -→ Mn 3+ + O 2-+ 1/2H 2 . At temperatures higher than 752 °C, a partial disorder between the Y and Z sites is observed from unit-cell parameters and mean bond distances, possibly caused by the inter-site exchange mechanism Y

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

“…5. The trend is very similar to that observed in both Fe-rich fluor-elbaite and Mn-bearing fluorelbaite (Celata et al 2021;Ballirano et al 2022).…”

“…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).…”

“…4. Table 3 lists the relevant parameters of the fitted functions that are compared to those of Fe-rich fluor-elbaite (Celata et al 2021) and Mnbearing fluor-elbaite (recalculated from the published data of Ballirano et al 2022). 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 both Fe-rich fluor-elbaite and Mn-bearing fluor-elbaite.…”