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

The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition X(Na0.41□0.35Ca0.24)Σ1.00 Y(Al1.81Li1.00Fe3+ 0.04Mn3+ 0.02Mn2+ 0.12Ti0.004)Σ3.00 ZAl6[T(Si5.60B0.40)Σ6.00O18](BO3)3(OH)3 W[(OH)0.50F0.13O0.37]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-LiAlSi2O6. A thermally induced oxidation of Mn2+ to Mn3+ 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: Mn2+ + (OH)– → Mn3+ + O2– + 1/2H2. 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 YLi + ZAl → ZLi + YAl.