The solubility of Zn 2+ in rs-NiO is much higher, namely, 29 mol% at 1100 K or 35 mol% at ~1500 K . At 1100 K, the solubility of Co 2+ in w-ZnO is 6.5 mol% while Zn 2+ solubility in rs-CoO is three times higher, reaching 22 mol% . Equilibrium T-x phase diagram of the FeO-ZnO system was previously studied at ambient pressure . Like other MeO-ZnO systems, this system is characterized by a large (18-80 mol% ZnO at 1100 K) area of co-existence of wurtzite and rock-salt solid solutions. At 0.1 GPa and 1100 K, the Mn 2+ solubility in w-ZnO reaches 25 mol% , which is the highest solubility value of all 3d transition metal ions. At the same time, Zn 2+ solubility in rs-MnO is not more than 9 mol%, which is the lowest solubility of Zn 2+ among 3d transition metal monoxides with rock-salt structure .