Crystal chemistry of the jarosite group of minerals - solid solution and atomic structures
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Authors
Basciano, Laurel C.
Date
2008-05-08T20:00:20Z
Type
thesis
Language
eng
Keyword
Jarosite , Solid solution , Rietveld refinement , X-ray diffraction
Alternative Title
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.
Description
Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2008-05-07 18:21:45.136
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