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    Crystal chemistry of the jarosite group of minerals - solid solution and atomic structures

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    Basciano_Laurel_C_200705_PhD.pdf (2.940Mb)
    Date
    2008-05-08
    Author
    Basciano, Laurel C.
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    Abstract
    The jarosite group of minerals is part of the alunite supergroup, which consists of

    more than 40 different mineral species that have the general formula AB3(TO4)2(OH,

    H2O)6. There is extensive solid-solution in the A, B and T sites within the alunite

    supergroup. Jarosite group minerals are common in acid mine waste and there is

    evidence of jarosite existing on Mars. Members of the jarosite - natrojarosite –

    hydronium jarosite (K,Na, H3O)Fe3(SO4)2(OH)6 solid-solution series were synthesized

    and investigated by Rietveld analysis of X-ray powder diffraction data. The synthesized

    samples have full iron occupancy, where in many previous studies there was significant

    vacancies in the B site. Well-defined trends can be seen in the unit cell parameters, bond

    lengths A – O and Fe - O across the solid-solution series in the synthetic samples. Based

    on unit cell parameters many natural samples appear to have full iron occupancy and

    correlate well with the synthetic samples from this study. In addition, the infrared spectra

    of the samples were analyzed. The atomic structure of ammoniojarosite,

    (NH4)Fe3(SO4)2(OH)6, has been solved using single-crystal X-ray diffraction to wR

    3.64% and R 1.4%. The atomic coordinates of the hydrogen atoms of the NH4 group

    have been located and it was found that the ammonium group has two different

    orientations with equal probability. Samples in the ammoniojarosite – hydronium jarosite

    solid-solution series were synthesized and analyzed using powder X-ray diffraction and

    Rietveld refinement. It was found that an incomplete solid-solution series exists between

    jarosite and plumbojarosite, Pb[Fe3(SO4)2(OH)6]2, based on experimental and

    mineralogical data. At the studied synthesis conditions, lead solubility in jarosite is

    extremely limited with occupancy of 2% in the potassium site. Increased Pb in the

    iv

    starting solution resulted in no increased substitution of Pb into jarosite, but an increased

    substitution of H3O+. The stable isotope (H) geochemistry of hydronium jarosite,

    (H3O,K)Fe3(SO4)2(O,OH)6, and the effect that the presence of hydronium in the crystal

    structure has on exchange rates of stable isotope values of jarosite with hydronium

    substitution has been investigated in this study.
    URI for this record
    http://hdl.handle.net/1974/1199
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    • Department of Geological Sciences and Geological Engineering Graduate Theses
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