Non-stoichiometric nickel arsenides in nature: The structure of orcelite, Ni5−xAs2 (x = 0.25), from the Bon Accord oxide body, South Africa

Abstract

The crystal structure of the mineral orcelite, a rare nickel arsenide reported in the literature with the formula Ni5−xAs2 (with x=0.23), was refined using intensity data collected from a crystal from the Bon Accord body, South Africa. This study revealed that the structure is hexagonal, space group P63mc, with a=6.7922(2), c=12.4975(5)Å, and V=499.31(3)Å3. The refinement of an anisotropic model led to an R index of 0.028 for 412 independent reflections. The orcelite structure can be described as a distorted variant of the Pd5Sb2 structure. The smaller As atoms are in the centres of distorted tetragonal antiprisms, formed by only Ni atoms. The coordination sphere is completed with two additional Ni atoms opposite to the rectangular faces. Electron microprobe data carried out on the same crystal used for the structural study point to the following formula [on the basis of Σ(As+Fe+Sb)=2]: Ni4.75(As1.93Fe0.05Sb0.02). According to the high-quality structure refinement, the minor elements were found to replace As in the structure. An atomic position for Ni was found to be partially occupied (75%), thus confirming the non-stoichiometry for the mineral orcelite previously reported in literature. Such a deviation from the stoichiometry could represent the driving force favouring disorder phenomena during the growth of the mineral.