Cation ordering in the annabergite-erythrite-hörnesite solid solutions

The solid solutions among annabergite [Ni3(AsO4)2·8H2O], erythrite [Co3(AsO4)2·8H2O], and hörnesite [Mg3(AsO4)2·8H2O] may control the arsenic concentrations in some mine wastes or natural systems rich in arsenic. These solid solutions are continuous and the divalent cations occur on two types of octahedral sites: the monomeric 2a sites and the dimeric 4g sites. The cation ordering influences the configuration entropy of mixing and hence, the thermodynamic properties, stability, and solubility of these solid solutions. In this work, cation ordering was investigated by constant-wavelength powder X-ray diffraction (for Ni-Mg and Co-Mg solid solutions) and by anomalous X-ray diffraction (for Ni-Co s.s.). For the Ni-Mg and Co-Mg systems, we found a strong preference of Mg2+ for the 4g site. In the Ni-Co series, the ordering is weak, with Co2+ being slightly preferred at the 2a site. The entropies of mixing, calculated from our data, are sufficiently large to retain negative Gibbs energies of mixing in these solid solutions, even though in some cases, the mixing enthalpies are positive. Hence, the mixing in these solutions is spontaneous and the cation uptake under natural conditions will be mostly dictated by the difference in the solubility products of the end members.