Second update on compounds with kröhnkite-type chains

The crystal structures of five new synthetic double salt dihydrates have been determined from single-crystal X-ray diffraction data. The following three selenate(VI), sulphate(VI) and chromate(VI) compounds all contain kröhnkite-[Na2Cu(SO4)2 · 2 H2O]-type infinite tetrahedral-octahedral chains: K2Zn(SeO4)2 · 2 H2O (a = 5.571(1), b = 6.586(1), c = 7.458(2) Å, α = 111.14(3), β = 96.35(3), γ = 106.15(3)°, V = 238.26(8) Å3, R(F) = 0.037, space group P-1, no. 2, Z = 1), K2Co(SO4)2 · 2 H2O (a = 5.327(1), b = 6.566(1), c = 7.256(2) Å, α = 110.74(3), β = 95.00(3), γ = 107.02(3)°, V = 221.72(8) Å3, R(F) = 0.037, space group P-1, no. 2, Z = 1), Rb2Cd(CrO4)2 · 2 H2O (a = 6.852(1), b = 7.706(2), c = 11.362(2) Å, α = 73.07(3), β = 75.11(3), γ = 69.63(3)°, V = 529.88(18) Å3, R(F) = 0.026, space group P-1, no. 2, Z = 2). The crystal structures of the two remaining compounds, the isotypic dichromates K2Co(Cr2O7)2 · 2 H2O and Cs2Cd(Cr2O7)2 · 2 H2O (a = 6.801(1)/7.368(1), b = 7.723(1)/8.051(2), c = 7.814(1) /8.047(2) Å, α = 89.23(3)/88.68(3), β = 81.05(3)/81.12(3), γ = 64.39(3)/63.81(3)°, V = 364.93(14)/422.66(16) Å3, R(F) = 0.022/0.025, space group P-1, no. 2, Z = 1) are closely related to those of kröhnkite-type compounds; the structure contains Cr2O7 groups assuming the same bridging role that XO4 groups have in kröhnkite-type oxysalts. Both dichromates are isotypic with their K—Zn- and K—Cd-analogues although the hydrogen bonding scheme in Cs2Cd(Cr2O7)2 · 2 H2O is completely different. The atomic arrangements of the five new compounds are described and unusual features are pointed out. Structural relations with other kröhnkite-type and -like minerals and compounds are discussed with respect to our previously proposed classification (Fleck et al., 2002; Fleck and Kolitsch, 2003). The compound Rb2Cd(CrO4)2 · 2 H2O represents both the first kröhnkite-type rubidium compound and the first chromate with structure type D. Cs2Cd(Cr2O7)2 · 2 H2O is the first caesium compound known to contain kröhnkite-like chains.