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

Jens Najorka,Mark A Sephton,James M T Lewis,John Spratt

doi:10.1007/s00269-016-0802-0

Jens Najorka, Mark A Sephton + Show 2 more

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https://doi.org/10.1007/s00269-016-0802-0

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Abstract

Na–H3O jarosite was synthesized hydrothermally at 413 K for 8 days and investigated using single-crystal X-ray diffraction (XRD) and electron microprobe analysis (EMPA). The chemical composition of the studied crystal is [Na0.57(3) (H3O)0.36 (H2O)0.07]A Fe2.93(3) (SO4)2 (OH)5.70 (H2O)0.30, and Fe deficiency was confirmed by both EMPA and XRD analysis. The single-crystal XRD data were collected at 298 and 102 K, and crystal structures were refined in space group $$ R\overline{3}m$$ . The room-temperature data match structural trends of the jarosite group, which vary linearly with the c axis. The low-temperature structure at 102 K shows an anisotropic decrease in the unit cell parameters, with c and a decreasing by 0.45 and 0.03 %, respectively. Structural changes are mainly confined to the A site environment. Only minor changes occur in FeO6 and SO4 polyhedra. The structure responds upon cooling by increasing bond length distortion and by decreasing quadratic elongation of the large AO12 polyhedra. The structural parameters at low temperature follow very similar patterns to structural changes that correspond to compositional variation in the jarosite group, which is characterised by the flexibility of AO12 polyhedra and rigidity of Fe(OH)4O2–SO4 layers. The most flexible areas in the jarosite structure are localized at AO12 edges that are not shared with neighbouring FeO6 octahedra. Importantly, for the application of XRD in planetary settings, the temperature-related changes in jarosite can mimic compositional change.

Highlights

Jarosite group minerals are members of the alunite supergroup, which has the general formula AB3(TO4)2(OH)6
It can be concluded that temperature- and compositionaldependent changes of bond lengths and distortion parameters follow similar patterns for all minerals in the jarosite group (Figs. 4, 5, 6)
Structural changes upon cooling resemble effects caused by the incorporation of smaller cations at the A site

Summary

Introduction

Jarosite group minerals are members of the alunite supergroup, which has the general formula AB3(TO4)2(OH). A range of cations can be incorporated into jarosite group minerals such as A = Na, K, Ag, Tl, H3O, NH4, Pb, B = Fe3+ and T = S Jarosite group minerals are mainly a product of chemical weathering. Low-temperature interaction of fluids with sulphide minerals in oxidizing and acidic environments are favourable formation conditions. On Earth, jarosite can be found in oxidation zones of sulphide-bearing ore deposits, in sediments and rocks associated with pyrite weathering, and in hydrothermally altered rocks (Dutrizac and Jambor 2000; Stoffregen et al 2000). Jarosite group minerals are known to precipitate in acid mine drainage environment Jarosite group minerals are known to precipitate in acid mine drainage environment (e.g. Hudson-Edwards 2003; Jamieson et al 2005)

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