• journal_title：European Journal of Mineralogy
• Contributor：Uwe KOLITSCH ; Michel FLECK
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2006/0018-0471
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：18
• issue：4
• firstpage：471
• section：Articles

As part of a comprehensive crystal-chemical study and classification of minerals and synthetic compounds with kröhnkite-type or -like chains of octahedra bridged by tetrahedra, the crystal structures of wendwilsonite from Schneeberg, Saxony, Germany, with composition Ca<sub>2sub>(Mg<sub>0.90sub>,Co<sub>0.10sub>)(AsO<sub>4sub>)<sub>2sub>·2H<sub>2sub>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 P2<sub>1sub>/c (no. 14), a = 5.806(1), b = 12.923(3), c = 5.628(1) Å β = 107.49(3)°, V = 402.75(14) Å<sup>3sup>, Z = 2; R(F) = 1.42%] and adopts type D in the classification of <span id="xref-ref-15-1" class="xref-bibr">Fleck et al. (2002b)span>; it is isotypic with the three arsenates brandtite, roselite, zincroselite, and the sulphate kröhnkite. Infinite kröhnkite-type tetrahedral-octahedral chains [(Mg,Co)(AsO<sub>4sub>)<sub>2sub>(H<sub>2sub>O)<sub>2sub>]<sup>2-sup> are separated by Ca<sup>2+sup> cations. A very strong hydrogen bond (O…O = 2.5294(18)Å) links adjacent chains.

Hydrothermally synthesised, orange AgSc(CrO<sub>4sub>)<sub>2sub>·2H<sub>2sub>O is triclinic [space group P1̄ (no. 2), a = 5.586(1), b = 6.054(1), c = 7.432(1)Å, α = 110.74(3), β = 90.21(3), γ = 117.34(3)°, V = 204.56(6) Å<sup>3sup>, Z = 1; R(F) = 3.48%] and represents a novel structure type, designated G, among compounds containing kröhnkite-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(CrO<sub>4sub>)<sub>2sub>·2H<sub>2sub>O also represents the first Ag and Sc compound based on a kröhnkite-type chain. The structure is based on infinite tetrahedral-octahedral chains [Sc(CrO<sub>4sub>)<sub>2sub>(H<sub>2sub>O)<sub>2sub>]<sup>2-sup> separated by Ag<sup>1+sup> cations. The Ag atom exhibits a [4+4]-coordination, with the AgO<sub>8sub> polyhedron closely resembling a strongly sheared cube.

Rb<sub>2sub>Cu(Cr<sub>2sub>O<sub>7sub>)<sub>2sub>·2H<sub>2sub>O and isotypic Cs<sub>2sub>Cu(Cr<sub>2sub>O<sub>7sub>)<sub>2sub>·2H<sub>2sub>O were both grown at room temperature as orange blades from slightly acidic aqueous solutions. They represent a new, triclinic structure type containing kröhnkite-like octahedral-tetrahedral chains based on bridging Cr<sub>2sub>O<sub>7sub> groups [space group P1̄ (no. 2), a = 7.272(1) / 7.462(2), b = 7.648(2) / 7.819(2), c = 7.772(2) / 7.847(2)Å, 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) Å<sup>3sup>, 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 kröhnkite-like chains is demonstrated.