Crystal structure refinement of duftite, PbCu(AsO4)(OH), from Grube Clara, Oberwolfach, Schwarzwald, Germany

The crystal structure of duftite, PbCu(AsO4)(OH), has been re-evaluated on the basis of X-ray diffraction data from a single crystal from Grube Clara, Germany. Duftite is orthorhombic with a = 7.7668(4), b = 9.2005(5), c = 5.9878(3) Å; V =427.88(4) Å3, non-centrosymmetric space group P212121, Z = 4. The structure refinement converges to a final R1 value of 0.0174 for 1252 reflections and allows an accurate description of the crystallographic features. The results obtained on the natural sample are compared with those available in the literature for this phase, regarding its synthetic counterpart. The crystal structure of duftite is confirmed and this work reports the first full anisotropic crystal structure refinement for this mineral, with the position of the hydrogen atom determined from the final ΔF map. Duftite is a member of the adelite-descloizite group with the Cu2+ Jahn-Teller active cation in distorted octahedral coordination. Individual bond lengths and other geometrical parameters are compared with those of other arsenates of this group

The crystal structure of hanksite, Na22K(CO3)2(SO4)9Cl, refined from high-resolution X-ray diffraction data

The crystal structure of hanksite, Na22K(CO3)2(SO4)9Cl, has been re-evaluated on the basis of high-resolution X-ray diffraction data (MoKα) from a single crystal from the type locality Searles Lake. Hanksite is hexagonal with a = 10.4896(3), c = 21.2415(6) Å, V = 2024.11(13) ų, space group P63/m (no. 176), Z = 2. Anisotropic structure refinement converges to final R1 of 4.0% for 6792 reflections with IO > 2σ(IO) in the θ range 2-55°. The structure of hanksite consists of two interconnected chains made of KO12 and NaO6 polyhedra extending along c and contains additional 6-fold and 7-fold Na-centered polyhedra (not involved in the formation of the chains) and CO3 and SO4 groups. On the basis of geometrical features and bond valence analyses, the coordination number for the K site has been assumed as twelve and the environment of the Na4-centred polyhedron has been described as a six-sided half-shell, rather than as an irregular octahedron