The analysis of Mg 3 NiO 4 suffers from errors two (different structure used in refinement than was predicted) and three (no evidence for cation order in the predicted structure). The reported compound is cubic, a variant of the rock salt structure, sometimes called Murdochite, with octahedrally coordinated metals; the Mn ions form a fcc arrangement, while the Mg and Ni ions, are disordered on a different site. There is also an additional cation vacancy compared with the parent rock salt structure. The A-lab structure, is exactly the same, except the Mg and Ni ions are now ordered, and the particular ordering reduces the symmetry to R 3m.