Two crystals of lindackerite, ideally (Cu,Co,Ni)Cu4(AsO4)2 (AsO3OH)2 . 9 H2O from Jáchymov (formerly St. Joachimsthal), Czech Republic, were studied and found to be essentially isostructural. The microprobe analysis of the first sample yielded (in wt%): As 29.66, Cu 27.85, Co 1.50, Ni 0.83,O 39.96, sum 99.79, (Cu0.43COo.26Nio.14) )Σo.83 Cu4.00 [(AsO4) 1.66(AsO3OH)2.34] .9.24H2O, P1̄, a, = 6.440(2), b = 8.065(3), c= 10.411(5) Å, α = 85.44(4)°, β = 79.38(3)°, γ = 84.65(3)°, V = 528.1(3) Å3, Z=1, Dcalc = 3.261 g/cm3. The structure was refined to Robs = 5.03 %. The second crystal was an end member, without any Co, Ni yielded (in wt%): As 30.66, Cu31.76, Co<0.01, Ni<O.01, O39.30, sum 101.75, Cu0.89 Cu4.00[(AsO4)1.78 (AsO3OH)2.22] .8.01 H2O, α = 6.415(1), b = 8.048(1), c = 10.332(1) Å, α = 85.41(1)†, β = 79.50(1)°, γ = 84.71(1)°, V = 521.2(1) Å3. Z = 1, Dcalc = 3.312 g/cm3 was refined to Robs= 5.29 %. The atomic arrangement in lindackerite is very similar to that of chudobaite and geigerite. There are three cationic positions, two As and 15 oxygen atoms. The bond-valence calculation reveals that one oxygen position represents OH group and 7 others water molecules. The substituting Co and Ni atoms enter presumably the special position at 1/2, O,O, denoted as M. Of other two positions Cul is five-coordinated in configuration of square pyramid, whereas Cu2 is octahedrally coordinated. Edge-sharing Cul pyramids and Cu2 octahedra form infinite bands parallel to [110]. The adjacent bands are linked together by two kinds of As-tetrahedra. As a result a layer parallel to (001) is formed. The adjacent layers are connected via M atoms. In addition, the M position is surrounded by eight half-occupied water molecules (Ow31-Ow34) in four independent positions. This arrangement can be interpreted as two interpenetrating octahedra (with common corners shared with As2 tetrahedra) turned by ∼-45°. In chudobaite and geigerite only one of these alternative octahedra exists and respective positions are fully occupied. One “zeolitic” water molecule Owl is located in the interlayer space and is half-occupied. Of other two water molecules Ow4 forms one of apices of the Cu2 octahedron, and Ow2 is located near the base of the Cul pyramid, but is rather distant (2.79(1) Å) to complete octahedral coordination.

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