The 8 Å crystal structure and new crystal chemical data of rouxelite from the Monte Arsiccio mine, Apuan Alps, Italy

Abstract

The crystal structure of rouxelite from the Monte Arsiccio mine, Italy, has been investigated using single-crystal X-ray diffraction (SCXRD) to clarify its crystallography and crystal chemistry. The structure is described in space group C-1, with lattice parameters a= 43.1883(12), b= 8.1037(2), c= 38.1470(10) Å, α= 96.001(2), β= 116.615(2), γ= 95.372(2)°, and V = 11721.7(6) Å3. The structure can be considered as being a twofold superstructure (doubled b cell parameter) of the C2 rouxelite structure previously reported from Buca della Vena mine. The asymmetric unit in the structure of rouxelite contains 53 cation sites and 66 anion sites. The metal sites are composed of 22 Pb positions, 28 Sb positions, one Hg position, and two Cu positions. Among the Pb sites, four are mixed with Tl, Sb, Ag, and As and two are split. Among the Sb sites, three Sb sites are mixed with Pb and As and three are split. The Hg position includes Ag, and two sulfur sites (S65 and S66) are partially occupied. Final refinement, performed as a twin with volume ratios of 0.5489 : 0.4510(14), resulted in an R1 value of 0.0855 for 55765 unique reflections. The crystal under investigation was an intergrowth with a second domain whose cell parameters correspond to those of launayite. The resulting structural formulae obtained from the SCXRD study for the unit cell is either Cu8Ag2.08Hg3.068Tl2Pb83.568As1.448Sb111.836S261.52 (for Z = 1, ch = 2.16) or Cu8Ag2.09Hg3.064Tl2Pb83.556As1.452Sb111.84S261.32O1.52 (for Z = 1, ch = −0.47) (O content could not be reliably determined), making the definition of an ideal formula difficult. Additionally, a substantial volume of new chemical data for rouxelite has been included, covering both the Monte Arsiccio mine and the neighbouring Buca della Vena occurrences, thereby enhancing the previously published data. The crystal chemistry, substitution mechanisms, and modular description of rouxelite as well as the modular relationship to other minerals are also addressed.