Method of preparing gamma ferric hydroxyoxide powder

A method of preparing gamma ferric hydroxyoxide which consists essentially of:

• reacting ferrous ions in an alkaline solution to produce a suspension of ferrous hydroxide,
• passing an oxidizing gas into said suspension in a first oxidizing step to an extent sufficient to cause nucleation of gamma ferric hydroxyoxide to occur, and until the ratio of ferric ions to the total amount of iron in solution is in the range from 0.3 to 0.48, and the ratio of hydroxyl ions to ferrous ions is less than one and
• reducing the oxidation rate of said gas after such nucleation occurs and said ratios are effected to a value of 1/5 to 1/50 of the rate in said first oxidizing step in a second oxidizing step to cause crystal growth to occur on the nuclei formed in said first oxidizing step without significantly increasing the amount of new nuclei formed, with the resultant crystals exhibiting a central rectangular shape without any significant vein shape.