Donharrisite, Ni3HgS3: Crystal structure and revision of the chemical formula

The crystal structure of the mineral donharrisite, a rare nickel-mercury-sulfide reported in the literature with the formula Ni8Hg3S9, was solved and refined using intensity data collected from a crystal from Leogang, Salzburg Province, Austria. This study revealed that the structure is monoclinic, space group C2/m, with a = 11.574(3), b = 6.899(2), c = 5.419(1) Å, β = 93.71(2)° and V = 431.8(2) Å3. The refinement of an anisotropic model led to an R index of 0.044 for 383 independent reflections. In the crystal structure there are two Hg sites, four Ni sites and 2 S, with one Hg and one Ni site which are half-occupied. As typical of many intermetallic compounds, the donharrisite structure is characterized by atoms forming complex polyhedra as Hg[S6Ni4], Hg[S4Ni3], Ni[S4Ni2], Ni[S3NiHg], Ni[S6Ni2Hg2], Ni[S4Ni3Hg2], showing exceedingly short bond distances. Electron microprobe analyses of the crystal used for the structural study led to the formula Ni3.03Hg1.01S2.96, on the basis of 7 atoms. On the basis of information gained from this characterization the crystal chemical formula was revised according to the structural results, yielding Ni3HgS3 with Z = 4 (with a calculated density of 7.27 g/cm3), instead of Ni8Hg3S9 with Z = 2 (with a calculated density of 5.18 g/cm3) as previously reported. Although neither reflection streaks nor structural disorder were observed for the selected donharrisite crystal, the possibility that the refined structure (with partially-occupied sites and exceedingly short bond distances) could be a superposition of different layers belonging to an OD (order-disorder) structure is also discussed.