Jarosite-butlerite intergrowths in non-stoichiometric jarosites: crystal chemistry of monoclinic natrojarosite-hydroniumjarosite phases

Monoclinic, non-stoichiometric natrojarosite-hydroniumjarosite solid solution phases have been synthesized hydrothermally over a range of temperatures, starting compositions and reaction times, and have been characterized using Rietveld refinement of synchrotron X-ray diffraction data, and chemical and thermal analyses. The H atom locations have been obtained from refinement of neutron diffraction data on a deuterated sample. The results confirm a direct relationship between the monoclinic distortion and the ordering of iron site vacancies in one of two independent iron sites. Ordering of iron vacancies gives rise to domains containing butlerite-like 7 angstrom chains of corner-connected octahedra and tetrahedra. The formation of these chains within (100) planes results in an expansion of the monoclinic a lattice parameter and a contraction of the c parameter relative to stoichiometric jarosites. The results support a recent model for iron deficiency, whereby an iron vacancy is compensated by the replacement of four coordinated OH(-) ions by H(2)O molecules, with one of the H(2)O molecules coming from deprotonation of H(3)O(+). The general formula, based on intergrowth of stoichiometric jarosite and non-stoichiometric, butlerite-like regions, is [(Na,H(3)O)Fe(3) (SO(4))(2)(OH)(6)](1-y)[H(2)O)Fe(2)(SO(4))(2)(OH)(2)(H(2)O)(4)](y).