Crystal structure refinement of akaganéite

Powder X-ray diffraction data and the Rietveld method have been used to refine the crystal structure of akaganéite, Fe_7.6Ni_0.4O_6.35(OH)_9.65Cl_1.25, 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.