Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Paolo Ballirano, Beatrice Celata, HenriK Skogby, Giovanni B. Andreozzi, Ferdinando Bosi

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

Journal of Geosciences, volume 67 (2022), issue 2, 151 - 161

DOI: http://doi.org/10.3190/jgeosci.347

The thermal behavior of a gem-quality purplish-red Mn-bearing elbaite from the Anjanabonoina pegmatite, Madagascar, with composition ^X(Na_0.41□_0.35Ca_0.24)_Σ1.00 ^Y(Al_1.81Li_1.00Fe³⁺_0.04Mn³⁺_0.02Mn²⁺_0.12Ti_0.004)_Σ3.00^ZAl₆[^T(Si_5.60B_0.40)_Σ6.00O₁₈](BO₃)₃(OH)₃ ^W[(OH)_0.50F_0.13O_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 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 &gama;-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⁺ + O^2- + 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.