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

• Published in: Mineralogical Magazine
• Main Authors: Rozhdestvenskaya IV, Mugnaioli E, Czank M, Depmeier W, Kolb U, Merlino S
• Other Authors: Rozhdestvenskaya, Iv, Mugnaioli, E, Czank, M, Depmeier, W, Kolb, U, Merlino, S
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2011
• Subjects: Sakha; Sakha Republic; Yakutia; Siberia
• Online Access: http://hdl.handle.net/11568/1131350; https://doi.org/10.1180/minmag.2011.075.6.2833

Charoite, ideally (K,Sr,Ba,Mn)(15-16)(Ca,Na)(32)[(Si(70)(O,OH)(180))](OH,F)(4)center dot nH(2)O, 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) angstrom, beta = 95.85(9)degrees, V = 4565(24) angstrom(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 1/2 e. The shifts involve a hybrid dreier quadruple chain, [Si(17)O(43)](18-) and a double dreier chain, [Si(6)O(17)](10-). In charoite-90 adjacent blocks are stacked without shifts. RI Mugnaioli, Enrico/E-6237-2011; Kolb, Ute/A-2642-2011