Reinvestigation of the crystal structure of lautite, CuAsS

The crystal structure of the mineral lautite (copper arsenic sulfide), CuAsS, previously described as either centrosymmetric [Pnma; Marumo & Nowacki (1964 ▶). Schweiz. Miner. Petro. Mitt. 44, 439–454] or noncentrosymmetric [Pna21; Craig & Stephenson (1965 ▶). Acta Cryst. 19, 543–547], was reinvestigated by means of single-crystal X-ray diffraction. The centrosymmetric structural model reported previously was confirmed, although with improved precision for the atomic coordinates and interatomic distances. Lautite shows a sphalerite-derivative structure with a linking of Cu[AsS3], As[CuAs2S] and S[Cu3As] tetrahedra. All atoms lie on special positions (Wyckoff position 4c, site symmetry m).


Comment
The crystal structure of lautite was solved by Marumo & Nowacki (1964) in the space group Pnma (R = 9.0%) and by Craig & Stephenson (1965) in the space group Pna2 1 (R = 13.7%) by means of photographic data and three-dimensional Patterson-function. The low quality of the structural data given by these authors, however, did not allow to obtain an anisotropic model of the structure. Nevertheless, the topologies and interatomic distances of both centrosymmetric and non-centrosymmetric models are very similar.
Although the structural results obtained by Craig & Stephenson (1965) indicate the acentricity of the structure of CuAsS, no clear crystal-chemical reason for the choice of a noncentrosymmetric arrangement was given. To help resolve the concerns relating to the structure of natural lautite, we present new crystal structure data for lautite from its type locality (i.e., Marienberg, Saxony, Germany).
The centrosymmetric structural model previously reported by Marumo & Nowacki (1964) was confirmed, although a higher precision of refinement was achieved (e.s.d. improved by a factor of two) and refinement with anisotropic displacement parameters could be performed (Fig. 1). All atoms lie on special positions (Wyckoff position 4c, site symmetry m).
Within the framework, the As atoms form zigzag As-As chains along [010] exhibiting As-As bond distances [2.4965 (8) Å] and angles [97.48 (4)°] resembling the covalent As-As linkage observed within the sheets of the crystal structure of arsenic (Wyckoff, 1963).

Experimental
A crystal was selected from a natural specimen belonging to the Mineralogical Collection of the Natural History Museum of Florence (catalogue number 44202/G).