Akaganeite

Akaganeite, also written as the deprecated Akaganéite, is a chloride-containing iron(III) oxide-hydroxide mineral, formed by the weathering of pyrrhotite (Fe1−xS), an iron-deficient sulfide. Akaganeite is often described as the β phase of anhydrous ferric oxyhydroxide FeOOH, but some chloride (or fluoride) anions are commonly included in the structure, so a more accurate formula is FeO0.833(OH)1.167Cl0.167. This formula derives from the more general expression FeO1-x(OH)1+xClx, where x = 1/6 ≃ 0.167. As the anion O2− is doubly negatively charged, to maintain the electroneutrality in the crystal lattice, the electrical charge deficit (2x) in O2− must be compensated by two monovalent anions, one OH− and one Cl−. Nickel may substitute for iron, yielding the more general formula (Fe3+,Ni2+)8(OH,O)16Cl1.25 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. These tunnels are partially occupied by chloride anions, which provide the crystal with its structural stability.