Mineralogical and chemical characteristics of some natural jarosites

Abstract

This paper presents a detailed study of the mineralogical, microscopic, thermal, and spectral characteristics of jarosite and natrojarosite minerals. Systematic mineralogic and chemical examination of a suite of 32 natural stoichiometric jarosite and natrojarosite samples from diverse supergene and hydrothermal environments indicates that there is only limited solid solution between Na and K at low temperatures, which suggests the presence of a solvus in the jarosite-natrojarosite system at temperatures below about 140°C. The samples examined in this study consist of either end members or coexisting end-member pairs of jarosite and natrojarosite. Quantitative electron-probe microanalysis data for several natural hydrothermal samples show only end-member compositions for individual grains or zones, and no detectable alkali-site deficiencies, which indicates that there is no hydronium substitution within the analytical uncertainty of the method. In addition, there is no evidence of Fe deficiencies in the natural hydrothermal samples. Hydronium-bearing jarosite was detected in only one relatively young supergene sample suggesting that terrestrial hydronium-bearing jarosites generally are unstable over geologic timescales.Unit-cell parameters of the 20 natural stoichiometric jarosites and 12 natural stoichiometric natrojarosites examined in this study have distinct and narrow ranges in the a-and c-cell dimensions. There is no overlap of these parameters at the 1r level for the two end-member compositions. Several hydrothermal samples consist of fine-scale (2-10 lm) intimate intergrowths of jarosite and natrojarosite, which could have resulted from solid-state diffusion segregation or growth zoning due to variations in the Na/K activity ratio of hydrothermal solutions. Published by Elsevier Ltd.

Crystallization of Jarosite with Variable Al3+ Content: The Transition to Alunite

“…Recently, it has also been found on Mars [18]. In nature, jarosite is said to be rarely found as the pure Fe end member and has some Al substitution [19,20]. In nature, however, many other substitutions are likely to be observed in the jarosite/alunite structures in addition to the Fe/Al substitution, as found by Scott [21,22].…”

Synthesis, characterization and thermochemistry of synthetic Pb–As, Pb–Cu and Pb–Zn jarosites

“…In nature end-members of the alunite group minerals are rare. More often, the minerals are nonstoichiometric and solid solutions between the end-members form; some authors call them near-end-members (Desborough et al, 2010) . Some crystals contain fine-scale chemical zoning (Desborough et al, 2010;Papike et al, 2006).…”

“…More often, the minerals are nonstoichiometric and solid solutions between the end-members form; some authors call them near-end-members (Desborough et al, 2010) . Some crystals contain fine-scale chemical zoning (Desborough et al, 2010;Papike et al, 2006). Considering this complexity, the thermodynamic data for the solid solutions measured in this work may be applicable to real minerals.…”

Single-crystal X-ray diffraction study of synthetic sodium–hydronium jarosite

“…Jarosite senso stricto [K jarosite, KFe 3 (SO 4 ) 2 (OH) 6 ] and Na jarosite [NaFe 3 (SO 4 ) 2 (OH) 6 ] are the most common jarosite species observed in nature (e.g. Desborough et al 2010). K-deficiency is required for the formation of Na jarosite and alkali deficiency for the formation of H 3 O jarosite [(H 3 O) Fe 3 (SO 4 ) 2 (OH) 6 ].…”