Multidimensional structural variation in the cyanotrichite family of merotypes: camerolaite-3b-F\bar 1.

A new superstructure of the mineral camerolaite, Cu ₆ Al ₃ (OH) ₁₈ (H ₂ O) ₂ [Sb(OH) ₆ ](SO ₄ ), has been refined in space group P\bar 1 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) Å ³ and Z = 1, with R ₁ = 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 [SO ₄ ] and [Sb(OH) ₆ ] polyhedra along rods that lie between the layers, and phase coupling between rods that maximizes the distance between [SO ₄ ] groups of adjacent rods along a and c directions. This arrangement suggests an alternative nearly orthogonal choice of unit cell in F\bar 1 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.