THE ATOMIC STRUCTURE OF (H3O)Fe3+(SO4)2 AND RHOMBOCLASE, (H5O2)Fe3+(SO4)2•2H2O

The atomic structure of (H3O)Fe3+(SO4)(2), an important phase in the system Fe-S-O-H, has been determined by the analysis of single-crystal X-ray-diffraction data. Within the unit cell of (H3O)Fe(SO4)(2), there are three layers consisting of Fe(SO4)(2) sheets parallel to (001) composed of FeO6 octahedra and SO4 tetrahedra sharing corners. The structure of (H3O)Fe3+(SO4)(2) is very similar to that of (H3O)Al(SO4)(2) except that it is distorted from the R (3) over bar arrangement of (H3O)Al(SO4)(2) to P (1) over bar symmetry. The phase (H3O)Fe3+(SO4)(2) has a layer spacing of 7.90 angstrom, which is slightly larger than the spacing for (H3O)Al(SO4)(2)(7.75 angstrom). This increase is a direct result of the mean Fe3+-O bond length (1.976 angstrom) being longer than the mean Al-O bond length (1.879 angstrom). The longer Fe3+-O bond length also causes the array of oxygen atoms formed by the apices of the sulfate groups that bond to the hydronium groups to be farther apart. The adjustment of the hydrogen bonding to this change results in the lower symmetry. The crystal structure of rhomboclase, (H5O2)Fe3+(SO4)(2)center dot 2H(2)O, has been refined, and the hydrogen positions have been determined for the first time. The model describing the disorder of the H5O2 molecule is confirmed.