On the akaganeite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts

The crystal structure of akaganéite and the akaganéite to hematite transition has been studied by means of conventional and synchrotron X-ray and neutron powder diffraction. The chemical formula of akaganéite can be written as FeO0.833(OH)1.167Cl0.167. The crystal structure does not contain free water.

Heating below 200 °C will not alter the akaganéite structure. Initial water loss can be attributed to a large amount of adsorbed water due to a very small particle size; 0.15 μm by 0.03 μm. Chloride is released from the structure only in connection with the transformation to hematite. Due to its stability, the presence of akaganéite does not in itself posses a threat to iron artifacts, but it is rather a symptom of the presence of high concentrations of chloride in an acidic environment.