Thomas Armbruster, Nicola Döbelin, Adolf Peretti, Detlef Günther, Eric Reusser, Bernard Grobéty
March 28, 2015
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[Al 9 O 18 ], was re-determined by single-crystal X-ray diffraction. Structure refinements were performed in space groups P6 3 /m and P6 3 . The centrosymmetric P6 3 /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 P6 3 model was rejected. A previous structural study claimed that painite was noncentrosymmetric and differed from the related structures of jeremejevite B 5 [⃞ 3 Al 6 (OH) 3 O 15 ] and fluoborite B 3 [Mg 9 (F,OH) 9 O 9 ] in having lower symmetry. The structure of painite comprises a framework of AlO 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 Ti4+ replacing octahedral Al leading to the formula Ca 0.77 Na 0.19 Al 8.80 Ti 0.19 Cr 0.03 V 0.01 Zr 0.94 Hf 0.01 B 1.06 O 18 .