akaganeite

Akaganeite is a ferric iron oxyhydroxide mineral that contains chloride in its crystal structure. Its common formulation is described as FeOOH with chloride (often written FeO(OH)·Cl), and it crystallizes in the tetragonal system as the β-FeOOH phase. The material forms as a corrosion product under oxidizing, chloride-rich conditions and is frequently found in rusts on steel exposed to seawater or concrete containing deicing salts; it is also encountered in some archaeological iron artifacts from chloride environments.

The crystal structure of akaganeite features tunnels along the c-axis in which chloride ions reside, giving the mineral distinctive chloride-rich channels within the FeOOH framework. This composition and structure make akaganeite a characteristic marker of chloride-accelerated corrosion processes.

Akaganeite is metastable and tends to transform over time or with changes in conditions. With aging, drying, or heating, it commonly converts to more stable iron oxyhydroxides such as goethite (α-FeOOH) or hematite (α-Fe2O3), often releasing chloride in the process.

In practical terms, the presence of akaganeite in rust indicates exposure to chloride-containing environments and can act as a reservoir for chloride ions within the corrosion product. This can contribute to long-term corrosion of embedded steel, as chloride ions may be released during moisture fluctuations. Identification typically relies on X-ray diffraction and electron microscopy, alongside complementary spectroscopic methods.

The name akaganeite is derived from Japanese, reflecting its discovery in Japan.