Essential features of the polytypic charoite-96 structure compared to charoite-90

Charoite, ideally (K,Sr,Ba,Mn) 15-16(Ca,Na) 32[(Si 70(O,OH) 180)](OH,F) 4•nH 2O, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystalline fibres. We report the structure of charoite-96 (a = 32.11(6), b = 19.77(4), c = 7.23(1) Å, β = 95.85(9)°, V = 4565(24) Å 3, space group P2 1/m), which was solved ab initio by direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined to R 1/wR 2 = 0.34/0.37. The structure of charoite-96 is related to that of the charoite-90, which was also solved recently. Both structures are composed of three different types of dreier silicate chains running along [001] and separated by ribbons of edge-sharing Ca-and Na-centred octahedra. In the structure of charoite-96, adjacent blocks formed by three different silicate chains and stacked along the x axis, are shifted by a translation of ½ c. The shifts involve a hybrid dreier quadruple chain, [Si 17O 43] 18- and a double dreier chain, [Si 6O 17] 10-. In charoite-90 adjacent blocks are stacked without shifts. © 2011 Mineralogical Society