Can. J. Chem. 53, 12 (1975). The possibility of forming spinel-type ferrites MFez04 from M(0H)z or MO and a-FeO(0H) (goethite) or a-Fez03 (hematite) under hydrothermal conditions up to 350" has been investigated experimentally for M = Mg, Ca, Mn, Co, Ni, Cu, and Zn. Formation ofMFez04 was observed for M = Mn (jacobsite), Co, Ni (trevorite), and Zn (franklinite), and was in general favored by high temperatures, high pH, and the presence of NaNO3 or, especially, NaCl in solution. a-FeO (0H) is more reactive than a-FezO3, but generally converts to the latter at 220-260°, independently of pH, in neutral or alkaline media. Interpretation of the results is complicated by the ionexchanging properties and varying degrees of crystallinity of M(OH)z, and by the tendency of M(OH)2 to decompose to MO or to be oxidized at the higher temperatures. Nevertheless, the [Tradult par le journal]