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|Title: ||Crystal chemistry and formation mechanism of non-stoichiometric monoclinic K-jarosites|
|Authors: ||Grey, IE|
|Issue Date: ||1-Apr-2013|
|Publisher: ||Mineralogical Society|
|Citation: ||Grey, I. E., Scarlett, N. V. Y., & Brand, H. E. A. (2013). Crystal chemistry and formation mechanism of non-stoichiometric monoclinic K-jarosites. Mineralogical Magazine, 77 (3), 249-268. doi:10.1180/minmag.2013.077.3.03|
|Abstract: ||Syntheses in acidified hydrothermal (HT) solutions (1 N H2SO4 or stronger) produce monoclinic non-stoichiometric K-jarosites which contain Fe-site vacancies with long-range order. Syntheses in non-acidified HT solutions produce rhombohedral K-jarosites which contain relatively large numbers of Fe-site vacancies with no long-range order. Increasing the [Fe]/[K] ratio, reaction temperature and reaction time in non-acidified solutions promotes the formation of monoclinic jarosites which contain Fe-site vacancies with short-range order. A structural model including details of the ordering of the Fe-site vacancies was obtained by refinement of single-crystal synchrotron data from one of the HT synthesis products; this model was used to refine synchrotron powder X-ray diffraction data from products synthesized at different reaction times, temperatures and [Fe]/[K] ratios. Thermal and chemical analyses are consistent with a model for non-stoichiometry in which domains of stoichiometric jarosite are intergrown with butlerite-like iron-deficient domains with a composition [Fe-2(SO4)(2)(OH)(2)(H2O)(4)]. It was found that heterogeneous nucleation of monoclinic jarosite on Si disks is preceded by the formation of an oriented film of Maus's Salt, K5Fe3O(SO4)(6)center dot 10H(2)O, as a precursor phase, and that this transforms topotactically into oriented jarosite, which contains butlerite-like layers parallel to the disk surface. Structural models for the transformation of Maus's Salt into jarosite are proposed.© 2013, Mineralogical Society.|
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