Kermesite, Sb2S2O: crystal structure revision and order-disorder interpretation.

Kermesite, Sb2S2O, is a desymmetrized order-disorder (OD) structure of layers. Two data sets were recorded using twinned crystals from Pezinok, Slovakia (named as Pz21, Pz24). The primitive unit cell is triclinic, P1, Z = 4, cell parameters are a = 8.1416 (3), b = 10.6968 (3), c = 5.7835 (2) Å, α = 102.758 (3), β = 110.657 (3), γ = 101.020 (3)°, R(obs) = 0.0243 (Pz21), and a = 8.1372 (2), b = 10.6969 (2), c = 5.7840 (1) Å, α = 102.787 (2), β = 110.606 (2), γ = 100.983 (2)°, R(obs) = 0.0321 (Pz24). The structure can also be described in the non-standard pseudo-monoclinic octuple (Z = 32), F-centered (Kupčík) cell with extra points in 1/4,1/4,0; 1/4,3/4,1/2; 3/4,1/4,1/2; 3/4,3/4,0, with parameters a = 21.6466 (9), b = 8.1416 (3), c = 20.3824 (9) Å, α = 90.079 (4), β = 101.985 (5), γ = 89.948 (4)° (Pz21), and a = 21.6558 (5), b = 8.1372 (2), c = 20.3859 (8) Å, α = 90.028 (3), β = 101.994 (3), γ = 89.986 (2)° (Pz24). The structure is built of layers parallel to the bc plane, stacked along the a vector of the octuple cell, composed of ribbons parallel to the b vector: (i) ribbon of two strips of SbO5 flattened quadrangular pyramids, sharing apical edges; (ii) ribbon of edge-sharing corrugated lozenges SbO3S. Basal S atoms of pyramids share corners of lozenges. Sb atoms are displaced out of coordination polyhedra into the inter-layer space. The OD layer comprises adjacent halves of the structure building layers. The layer group is A(1)2/m1, the protocell is defined by b, c, (a/4)sin β. The MDO1 (4A) polytype is generated by repetition of the t(1,1/4,0) [or alternatively t(1,-1/4,0)] translation. The co-existence of two kinds of domains give rise to the twinning. The twin operation is 2[010], twin index 2. The total continuation of [. a2 .] generates the MDO2 (2M) polytype, space group A12/a1. Simulated and real diffraction patterns are presented. The important values (edges, angles) and displacements of atoms due to the desymmetrization were evaluated. The comparison with structures of stibnite, bismuthinite and aikinite is added.