The crystal structure of painite CaZrB[[Al.sub.9][O.sub.18]] revisited

The crystal structure of the rare hexagonal mineral painite [a = 8.724(1), c = 8.464(2) [Angstrom]] from Mogok (Myanmar), with the ideal composition CaZrB[[Al.sub.9][O.sub.18]], was re-determined by single-crystal X-ray diffraction. Structure refinements were performed in space groups P[6.sub.3]/m and P[6.sub.3]. The centrosymmetric P[6.sub.3]/m model yielded excellent agreement ([R.sub.1] = 1.44%, 1189 reflections > 2[alpha] [I.sub.obs], 54 parameters) with the observed diffraction data without any unusual atomic displacement parameters, thus the acentric P[6.sub.3] model was rejected. A previous structural study claimed that painite was noncentrosymmetric and differed from the related structures of jeremejevite [B.sub.5][[cube root of (term)].sub.3][Al.sub.6][(OH).sub.3][O.sub.15]] and fluoborite [B.sub.3][[Mg.sub.9][(F,OH).sub.9][O.sub.9]] in having lower symmetry. The structure of painite comprises a framework of Al[O.sub.6] 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 [Ti.sup.4+] replacing octahedral Al leading to the formula [Ca.sub.0.77] [Na.sub.0.19][Al.sub.8.80][Ti.sub.0.19][Cr.sub.0.03][V.sub.0.01] [Zr.sub.0.94][Hf.sub.0.01][B.sub.1.06][O.sub.18].