Powder X-ray diffraction data and the Rietveld method have been used to refine the crystal structure of akaganéite, Fe7.6Ni0.4O6.35(OH)9.65Cl1.25, that formed as a corrosion crust on the Camp del Cielo meteorite. The study confirms the hollandite-like structure. The symmetry, however, is not tetragonal as previously reported, but is monoclinic (I2/m) with: a = 10.600(2) Å, b = 3.0339(5) Å, c = 10.513(2) Å, β = 90.24(2)°. The monoclinic symmetry explains previous Mössbauer results showing two distinct Fe octahedral sites. The Cl ions partially fill the tunnels and probably are essential for the formation of the phase. Structure-energy calculations were used to determine likely H positions.

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