Multidimensional structural variation in the cyano­trichite family of merotypes: camerolaite-3b-F

A new superstructure of the mineral camerolaite, Cu₆Al₃(OH)₁₈(H₂O)₂[Sb(OH)₆](SO₄), has been refined in space group P with unit-cell parameters a_P = 7.7660 (16), b_P = 8.759 (4), c_P = 11.306 (2) Å, α_P = 108.67 (4), β_P = 83.41 (3), γ_P = 126.64 (2)°, V = 581.6 (3) ų and Z = 1, with R₁ = 0.0951 (all data). This is the first refined example of a cyano­trichite-group mineral in which long-range order of interlayer anions produces a superstructure along b, although diffuse scattering has previously been reported that corresponds to short-range order. Though the structure shares with other members of the cyano­trichite group a structural unit in which ribbons of edge-sharing Cu and Al octahedra form layers || (001), the superstructure arises from regular alternation of [SO₄] and [Sb(OH)₆] polyhedra along rods that lie between the layers, and phase coupling between rods that maximizes the distance between [SO₄] groups of adjacent rods along a and c directions. This arrangement suggests an alternative nearly orthogonal choice of unit cell in F with parameters a_F = 12.473 (26), b_F = 8.759 (4), c_F = 21.476 (7) Å, α_F = 85.94 (4), β_F = 95.91 (5), γ_F = 92.34 (9)°, which is recommended for comparing this structural variety of camerolaite with other members of the group.