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Minerals 2017, 7(6), 90;

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

Curtin Institute of Functional Molecules and Interfaces, Department of Chemistry, Curtin University, GPO Box U1987, Perth WA 6845, Australia
Academic Editor: Denis Gebauer
Received: 28 March 2017 / Revised: 29 May 2017 / Accepted: 29 May 2017 / Published: 1 June 2017
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This study focused on the formation of the jarosite-alunite solid solution at relatively low temperature, 90 °C. It was found that the transition from jarosite to alunite results in significant changes in the powder X-ray diffraction pattern, the infrared spectrum and thermal behavior when the degree of substitution reached ≥50%. The initial Al/(Al + Fe) in solution, however, required to achieve these substitution levels in the solid is ≥90%. The morphology shows that the faceted jarosite form goes through an intergrown transition to a spherical morphology of pure alunite. This morphology has not been previously observed for alunite and most likely reflects the formation temperature. Rietveld analysis shows that the a lattice parameter obeys Vegard’s Rule while the c lattice parameter behavior is more complex. Empirical modelling of the incorporation of Fe into alunite supports the general trends found in the X-ray diffraction data for the behaviour of the a-axis with Al/Fe content. The dehydration of the Al3+ ion could be a significant contribution to the activation energy barrier to alunite formation as found for other minerals. Finally, dynamic light scattering showed that the nucleation behavior for jarosite and Fe-containing alunite are significantly different. Alunite appears to nucleate continuously rather than in a single nucleation event. View Full-Text
Keywords: crystal growth; nucleation; infrared spectroscopy; XRD data; jarosite; alunite crystal growth; nucleation; infrared spectroscopy; XRD data; jarosite; alunite

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Jones, F. Crystallization of Jarosite with Variable Al3+ Content: The Transition to Alunite. Minerals 2017, 7, 90.

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