Akaganeite, also written as the deprecated Akaganéite,[2] is a chloride-containing iron(III) oxide-hydroxide mineral, formed by the weathering of pyrrhotite (Fe1−xS).

Akaganeite is often described as the β phase of anhydrous ferric oxyhydroxide FeOOH, but some chloride (or fluoride) ions are normally included in the structure,[3] so a more accurate formula is FeO0.833(OH)1.167Cl0.167.

[4] Nickel may substitute for iron, yielding the more general formula (Fe3+,Ni2+)8(OH,O)16Cl1.25[5] Akaganeite has a metallic luster and a brownish yellow streak.

Its crystal structure is monoclinic and similar to that of hollandite BaMn8O16, characterised by the presence of tunnels parallel to the c-axis of the tetragonal lattice.

[7][8] Akaganeite has also been found in widely dispersed locations around the world and in rocks from the Moon that were brought back during the Apollo Project.