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 compositionX(Na0.41 □0.35 Ca0.24)Σ1.00 Y (Al1.81 Li1.00 Fe3+0.04Mn3+0.02Mn2+0.12Ti0.004)Σ3.00Z Al6 [T(Si5.60 B0.40)Σ6.00 O18 ](BO3)3 (OH)3 W [(OH)0.50 F0.13 O0.37 ]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 Mn-bearing 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-LiAlSi2 O6 . 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 mechanismYLi +ZAl →ZLi +YAl.