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Non-stoichiometric (synthetic) jarosites: crystal chemistry

Posted by Uwe Kolitsch  
Non-stoichiometric (synthetic) jarosites: crystal chemistry
December 07, 2011 10:01PM
I. E. Grey, N. V. Y. Scarlett, P. Bordet, and H. E. A. Brand (2011): Jarosite–butlerite intergrowths in non-stoichiometric jarosites: crystal chemistry of monoclinic natrojarosite–hydroniumjarosite phases. Mineral. Mag. 75, 2775-2791.

Monoclinic, non-stoichiometric natrojarosite–hydroniumjarosite solid solution phases have been synthesized hydrothermally over a range of temperatures, starting compositions and reaction times, and have been characterized using Rietveld refinement of synchrotron X-ray diffraction data, and chemical and thermal analyses. The H atom locations have been obtained from 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 Å chains of corner-connected octahedra and tetrahedra. The formation of these chains within (100) planes results in an expansion of the monoclinic a lattice parameter and a contraction of the c parameter relative to stoichiometric jarosites. The results support a recent model for iron deficiency, whereby an iron vacancy is compensated by the replacement of four coordinated OH− ions by H2O molecules, with one of the H2O molecules coming from deprotonation of H3O+. The general formula, based on intergrowth of stoichiometric jarosite and non-stoichiometric, butlerite-like regions, is <(Na,H3O)Fe3(SO4)2(OH)6>1–y<(H2O)Fe2(SO4)2(OH)2(H2O)4>y.


Edited 1 time(s). Last edit at 12/07/2011 10:03PM by Uwe Kolitsch.

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