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

摘要

Monoclinic, non-stoichiometric natrojarosite-hydroniumjarosite solid solution phases have been synthesizedhyd rothermally over a range of temperatures, starting compositions andreaction times, and have been characterized using Rietveld refinement of synchrotron X-ray diffraction data, and chemical andthermal analyses. The H atom locations have been obtainedfrom 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 A chains of cornerconnected octahedra and tetrahedra. The formation of these chains within (100) planes results in an expansion of the monoclinic a lattice parameter anda contraction of the c parameter relative to stoichiometric jarosites. The results support a recent model for iron deficiency, whereby an iron vacancy is compensatedby the replacement of four coordinatedOH ~- ions by H_2O molecules, with one of the H_2O molecules coming from deprotonation of H_3O~+. The general formula, basedon intergrowth of stoichiometric jarosite andnon-stoichiometric, butlerite-like regions, is [(Na,H_3O)Fe_3 (SO_4)_2(OH)_6]_(1-y)[(H_2O)Fe_2(SO_4)_2(OH)_2(H_2O)_4]_y.