Crystal chemistry of the natrojarosite-jarosite and natrojarosite-hydronium jarositesolid-solution series: A synthetic study with full Fe site occupancy

摘要

Members of the natrojarosite-hydronium jarosite [(Na,H_3O)Fe_3(SO_4)_2(OH)_6] and jarosite-natrojarosite[(K,Na)Fe_3(SO_4)_2(OH)_6] solid-solution series were synthesized and investigated by Rietveld analysis ofX-ray powder diffraction data. The synthesized samples have full Fe occupancy, where in many previousstudies there were significant vacancies in the B site. Well-defined trends can be seen in the unit-cellparameters across the solid-solution series in the synthetic samples. The majority of the samples in thisstudy were directly synthesized under hydrothermal conditions at 140 °C. End-member natrojarosite wassynthesized using a two-step method, where the initial sample was heated in a 1.0 m H_2SO_4-0.5Na_4solution at 200 °C for 3 days, yielding a sample with 100% Na occupancy. Many of the samples wereinitially zoned and required grinding and re-heating in the reactant solution for homogenization. Sub-stitution of H30 and K into natrojarosite changes unit-cell parameters in a linear fashion. The unit-cellparameters presented here are significantly different than the majority of previous studies on syntheticsamples, as samples in the current study have full Fe occupancy and the Na-K jarosite series has noH30 substitution in the A site. Substitution in the A site mainly affects unit-cell parameter c with littlechange in a. As Na occupancy increases there is a decrease in A-O2 and A-O3 distances and a consequentincrease in Fe-O2 and Fe-O3 distance leading to an overall decrease in unit-cell parameter c in both theNa-H_3Oand Na-K jarosite series. The synthetic samples are compared to natural from minewaste deposits in Rio Tinto (Huelva, Spain), Ely Mine (Vermont), and a mineral collecting locality nearSharbot Lake (Ontario), as well as natural and synthetic samples documented in the literature. Based onunit-cell parameters many of the natural samples appear to have full Fe occupancy and correlate wellwith the synthetic samples from this study. The infrared spectra of the samples were analyzed, and thereis a gradual change in the spectral features across the solid-solution series between end-members. Theresults from this study will aid in the interpretation of the possible chemical compositions of naturaljarosite group minerals in mine waste and on Mars.