The structure of charoite, (K,Sr,Ba,Mn)_15–16(Ca,Na)₃₂[(Si₇₀(O,OH)₁₈₀)](OH,F)_4.0.<i>n</i>H₂O, solved by conventional and automated electron diffraction

Abstract

Abstract Charoite, ideally (K,Sr,Ba,Mn) 15–16 (Ca,Na) 32 [(Si 70 (O,OH) 180 )](OH,F) 4.0 · n H 2 O, a rare mineral from the Murun massif in Yakutiya, Russia, was studied using high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray spectroscopy, precession electron diffraction and the newly developed technique of automated electron-diffraction tomography. The structure of charoite ( a = 31.96(6) Å, b = 19.64(4) Å, c = 7.09(1) Å, β = 90.0(1)°, V = 4450(24) Å 3 , space group P 2 1 / m ) was solved ab initio by direct methods from 2878 unique observed reflections and refined to R 1 / wR 2 = 0.17/0.21. The structure can be visualized as being composed of three different dreier silicate chains: a double dreier chain, [Si 6 O 17 ] 10– ; a tubular loop-branched dreier triple chain, [Si 12 O 30 ] 12– ; and a tubular hybrid dreier quadruple chain, [Si 17 O 43 ] 18– . The silicate chains occur between ribbons of edge-sharing Ca and Na-octahedra. The chains of tetrahedra and the ribbons of octahedra extend parallel to the z axis. K + , Ba 2+ , Sr 2+ , Mn 2+ and H 2 O molecules lie inside tubes and channels of the structure. On the basis of microprobe analyses and occupancy refinement of the cation sites, the crystal chemical formula of this charoite can be written as ( Z = 1): (K 13.88 Sr 1.0 Ba 0.32 Mn 0.36 ) Σ15.56 (Ca 25.64 Na 6.36 ) Σ32 [(Si 6 O 11 (O,OH) 6 ) 2 (Si 12 O 18 (O,OH) 12 ) 2 (Si 17 O 25 (O,OH) 18 ) 2 ](OH,F) 4.0 ·3.18H 2 O.