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
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