Natural and synthetic compounds with kröhnkite-type chains. An update

Abstract

Abstract The crystal structures of five new triclinic double salt dihydrates have been determined from single-crystal X-ray diffraction data. The following four compounds all contain kröhnkite-[Na2Cu(SO4)2·2 H2O]-type tetrahedral-octahedral chains: the three isotypic synthetic compounds K2Mg(CrO4)2·2 H2O (a = 5.674(1), b = 6.462(1), c = 7.517(2) Å, α = 110.38(3), β = 95.24(3), γ = 109.86(3)°, V = 236.0(1) Å3, R(F) = 0.036, space group P1̅, no. 2, Z = 1), K2Mg(MoO4)2·2 H2O (a = 5.884(1), b = 6.491(1), c = 7.700(1) Å, α = 111.67(2), β = 96.59(2), γ = 108.62(2)°, V = 249.8(1) Å3, R(F) = 0.019, space group P1̅, no. 2, Z = 1) and K2Mn(SeO4)2·2 H2O (a = 5.674(1), b = 6.608(1), c = 7.523(2) Å, α = 110.31(3), β = 95.69(3), γ = 108.35(3)°, V = 244.10(9) Å3, R(F) = 0.024, space group P1̅, no. 2, Z = 1), and the mineral messelite Ca2(Fe2+,Mn2+,Mg)(PO4)2·2 H2O (a = 5.480(1), b = 5.759(1), c = 6.569(1) Å, α = 90.18(3), β = 102.62(3), γ = 108.45(3)°, V = 191.3(1) Å3, R(F) = 0.022, space group P1̅, no. 2, Z = 1). The fifth compound, the synthetic dichromate K2Zn(Cr2O7)2·2 H2O (a = 6.794(1), b = 7.735(2), c = 7.834(2) Å, α = 88.97(3), β = 80.90(3), γ = 64.57(3)°, V = 366.5(2) Å3, R(F) = 0.036, space group P1̅, no. 2, Z = 1) is closely related to the kröhnkite-group; it contains Cr2O7 groups assuming the same bridging role that XO4 groups have in kröhnkite-type oxysalts. The atomic arrangements of the title compounds are described and structural relations with other kröhnkite-type and -like compounds are discussed. In addition, related yavapaiite-type sheet structures of AM′(XO4)2 compounds are briefly reviewed, including the rod group symmetry of the underlying kröhnkite-chain building unit. Twelve different space groups have been found for these sheet structures. The previously reported space group symmetries for α-NH4Fe(CrO4)2 (P21) and CsTa(PO4)2 (P1̅) are obviously incorrect and should be revised to P21/n and C2/m, respectively.