Summary: A new mineral, akaganéite, has been found in limonite deposits and has been shown to be identical with synthetic β-FeOOH. It contains no chlorine. The micro-morphology of β-FeOOH as it occurs as akaganéite and as it is produced by a solid-state transformation from FeCl₂.4H₂O , by the hydrolysis of dilute FeCl₃ solutions, and by the hydrolysis of very concentrated FeCl₃ a solutions, has been studied, chiefly by electron microscopy and diffraction. Some modes of colloidal association of the particles of β-FeOOH are described.