Third update on compounds with kröhnkite-type chains: The crystal structure of wendwilsonite [Ca2Mg(AsO4)2 ·2H2O] and the new triclinic structure types of synthetic AgSc(CrO4)2·2H2O and M2Cu(Cr2O7)2·2 H2O (M = Rb, Cs)
- Author(s)
- Uwe Kolitsch, Michel Fleck
- Abstract
As part of a comprehensive crystal-chemical study and classification of minerals and synthetic compounds with krošhnkite-type or -like chains of octahedra bridged by tetrahedra, the crystal structures of wendwilsonite from Schneeberg, Saxony, Germany, with composition Ca2(Mg0.90,Co0.10)(AsO4) 2ž2H2O, and of three new synthetic double-metal dichromate(VI) dihydrates have been determined from single-crystal X-ray diffraction data. Wendwilsonite is monoclinic [space group P21/c (no. 14), a = 5.806(1), b = 12.923(3), c = 5.628(1) A°, ß = 107.49(3)°, V = 402.75(14) A°3, Z = 2; R(F) = 1.42%] and adopts type D in the classification of Fleck et al. (2002b); it is isotypic with the three arsenates brandtite, roselite, zincroselite, and the sulphate krošhnkite. Infinite krošhnkite-type tetrahedral-octahedral chains [(Mg,Co)(AsO4)2(H2O) 2]2- are separated by Ca2+ cations. A very strong hydrogen bond (O?O = 2.5294(18) A°) links adjacent chains. Hydrothermally synthesised, orange AgSc(CrO4)2ž2H2O is triclinic [space group P1¯ (no. 2), a = 5.586(1), b = 6.054(1), c = 7.432(1) A°, a = 110.74(3), ß = 90.21(3), ? = 117.34(3)°, V = 204.56(6) A°3, Z = 1; R(F) = 3.48%] and represents a novel structure type, designated G, among compounds containing krošhnkite-type chains. The structure is metrically pseudo-monoclinic and closely related to those of the monoclinic subtypes F1 (C2/c) and F2 (C2/m). AgSc(CrO4)2ž2H2O also represents the first Ag and Sc compound based on a krošhnkite-type chain. The structure is based on infinite tetrahedral-octahedral chains [Sc(CrO4)2(H2O)2] 2- separated by Ag1+ cations. The Ag atom exhibits a [4+4]-coordination, with the AgO8 polyhedron closely resembling a strongly sheared cube. Rb2Cu(Cr2 O7)2ž2H2O and isotypic Cs2Cu(Cr2O7)2ž2 H2O were both grown at room temperature as orange blades from slightly acidic aqueous solutions. They represent a new, triclinic structure type containing krošhnkite-like octahedral-tetrahedral chains based on bridging Cr2O7 groups [space group P1¯ (no. 2), a = 7.272(1) / 7.462(2), b = 7.648(2) / 7.819(2), c = 7.772(2) / 7.847(2) A°, a = 91.89(3) / 93.19(3), ß = 99.72(3) / 99.57(3), ? = 115.79(3) / 116.34(3)°, V = 380.84(15) / 400.23(15) A°3, Z = 1; R(F) = 3.03 / 2.99%]. The Cu atoms show a Jahn-Teller-distorted 'octahedral' coordination environment, approaching an unusual [2+2+2]-coordination. Hydrogen bonding is weak. A distinct positive correlation between the size of the alkali cation and the Cr-O-Cr angle in the few known alkali metal dichromates with krošhnkite-like chains is demonstrated. Œ 2006 E. Schweizerbart'sche Verlagsbuchhandlung.
- Organisation(s)
- Department of Mineralogy and Crystallography
- Journal
- European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
- Volume
- 18
- Pages
- 471-482
- No. of pages
- 12
- ISSN
- 0935-1221
- DOI
- https://doi.org/10.1127/0935-1221/2006/0018-0471
- Publication date
- 2006
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105113 Crystallography
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/f200b415-9ee2-4829-bcfd-c27a841f6aa1