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(Na0.410.35Ca0.24)sigma 1.00Y(Al1.81Li1.00Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)sigma 3.00ZAl6[T(Si5.60B0.40)sigma 6.00O18](BO3)3(OH)3 W[(OH)0.50F0.13O0.37]sigma 1.00 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 degrees C. The first occurrence of mullite diffraction peaks allowed us to constrain the breakdown temperature of Mn-bearing elbaite at ambient pressure, at 825 degrees C. The breakdown products from the HT-pXRD experiments were cooled down to ambient temperature and identified via pXRD, represented by B-mullite and gamma-LiAlSi2O6. A thermally induced oxidation of Mn2+ to Mn3+ was observed with both in-situ and ex-situ techniques; it started at 470 degrees C and is assumed to be counterbalanced by deprotonation, according to the equation: Mn2++ (OH)--iota Mn3+ + O2- + 1/2H2. At temperatures higher than 752 degrees 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-iota ZLi + YAl.