Third update on compounds with krohnkite-type chains: the crystal structure of wendwilsonite [Ca2Mg(AsO4)22H2O] and the new triclinic structure types of synthetic AgSc(CrO4)22H2O and M2Cu(Cr2O7)22H2O (M = Rb, Cs)

Abstract

As part of a comprehensive crystal-chemical study and classification of minerals and synthetic compounds with krohnkite-type or -like chains of octahedra bridged by tetrahedra, the crystal structures of wendwilsonite from Schneeberg, Saxony, Germany, with composition Ca 2 (Mg 0.90 ,Co 0.10 )(AsO 4 ) 2 ·2H 2 O, 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 P 2 1 / c (no. 14), a = 5.806(1), b = 12.923(3), c = 5.628(1) A β = 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 krohnkite. Infinite krohnkite-type tetrahedral-octahedral chains [(Mg,Co)(AsO 4 ) 2 (H 2 O) 2 ] 2- are separated by Ca 2+ cations. A very strong hydrogen bond (O…O = 2.5294(18)A) links adjacent chains. Hydrothermally synthesised, orange AgSc(CrO 4 ) 2 ·2H 2 O is triclinic [space group P 1 (no. 2), a = 5.586(1), b = 6.054(1), c = 7.432(1)A, α = 110.74(3), β = 90.21(3), γ = 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 krohnkite-type chains. The structure is metrically pseudo-monoclinic and closely related to those of the monoclinic subtypes F1 ( C 2/ c ) and F2 ( C 2/ m ). AgSc(CrO 4 ) 2 ·2H 2 O also represents the first Ag and Sc compound based on a krohnkite-type chain. The structure is based on infinite tetrahedral-octahedral chains [Sc(CrO 4 ) 2 (H 2 O) 2 ] 2- separated by Ag 1+ cations. The Ag atom exhibits a [4+4]-coordination, with the AgO 8 polyhedron closely resembling a strongly sheared cube. Rb 2 Cu(Cr 2 O 7 ) 2 ·2H 2 O and isotypic Cs 2 Cu(Cr 2 O 7 ) 2 ·2H 2 O were both grown at room temperature as orange blades from slightly acidic aqueous solutions. They represent a new, triclinic structure type containing krohnkite-like octahedral-tetrahedral chains based on bridging Cr 2 O 7 groups [space group P 1 (no. 2), a = 7.272(1) / 7.462(2), b = 7.648(2) / 7.819(2), c = 7.772(2) / 7.847(2)A, α = 91.89(3) / 93.19(3), β = 99.72(3) / 99.57(3), γ = 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 krohnkite-like chains is demonstrated.