Production of cobalt-doped ferrimagnetic iron oxides

A process for the production of a cobalt-doped iron oxide acicular pigment protected at least partly against sintering comprising forming a suspension of FeOOH seeds in water containing zinc, phosphate and ferrous ions, introducing cobalt ions, adding alkali and air to precipitate ferric and cobalt ions onto the seeds, adding a phosphate solution in quantity of from about 0.2 to 1% by weight of P.sub.2 O.sub.5 into the water containing the seeds which are now pigment, separating the pigment from the suspension, reducing the pigment to Fe.sub.3 O.sub.4, oxidizing the Fe.sub.3 O.sub.4 pigment in an oxygen-containing gas to bring the FeO content in the pigment to about 16 to 24% by weight, and heat treating the oxidized pigment under inert conditions at a temperature of about 600.degree. to 800.degree. C., the amount of zinc, cobalt and phosphate ions present being such as to impart to the end product a content by weight of about 0.7 to 2.4% of cobalt, about 0.1 to 2% of zinc, and about 0.1 to 2% of phosphorus expressed as P.sub.2 O.sub.5, the coercive force of the resulting pigment amounting to at least about 425 Oe+175 Oe/%, by weight, of Co.