The crystal structure of painite CaZrB[Al9O18] revisited.

The crystal structure of the rare hexagonal mineral painite [a =18.724(1), c = 8.464(2) Å] from Mogok (Myanmar), with the ideal composition CaZrB[Al₉O₁₈, was re-determined by single-crystal X-ray diffraction. Structure refinements were performed in space groups P6₃/m and P6. The centrosymmetric P6₃/m model yielded excellent agreement (R₁ = 1.44%, 1189 reflections > 2σ I_obs, 54 parameters) with the observed diffraction data without any unusual atomic displacement parameters, thus the acentric P63 model was rejected. A previous structural study claimed that painite was non-centrosymmetric and differed from the related structures of jeremejevite B₅[〉₃Al₆(OH)₃O₁₅] and fluoborite B₃ [Mg₉(F,OH)₉, O₉] in having lower symmetry. The structure of painite comprises a framework of AlO₆ octahedra that features two types of channels parallel to the c axis. One channel has a trigonal cross-section and is occupied by threefold coordinated B and Zr in sixfold prismatic coordination. The other channel has a hexagonal cross-section and is occupied by Ca. Chemical analysis by laser-ablation inductively-coupled plasma-mass spectrometry indicated that the crystal studied has significant substitution of Na for Ca (ca. 20%) charge-balanced by Ti⁴⁺ replacing octahedral Al leading to the formula Ca_0.77Na_0.19Al_8.80Ti_0.19Cr_0.03V_0.01Zr Hf_0.01B_1.06O₁₈. [ABSTRACT FROM AUTHOR]