Abstract

The crystal structure of the rare hexagonal mineral painite [a = 8.724(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 P63. The centrosymmetric P63/m model yielded excellent agreement (R1 = 1.44%, 1189 reflections > 2σ Iobs, 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 noncentrosymmetric and differed from the related structures of jeremejevite B5[□3Al6(OH)3O15] and fluoborite B3[Mg9(F,OH)9O9] 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 Ca0.77Na0.19Al8.80Ti0.19Cr0.03V0.01Zr0.94Hf0.01B1.06O18.

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