Browsing by Author "Rintoul, L"

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    The crystal structure and vibrational spectroscopy of jarosite and alunite minerals
    (GeoScience World, 2013-10-01) Spratt, HJ; Rintoul, L; Avdeev, M; Martens, WN
    The alunite supergroup of minerals is a large hydroxy-sulfate mineral group, which has seen renewed interest following their discovery on Mars. Numerous reviews exist concerning nomenclature, formation, and natural occurrence of this mineral group. Sulfate minerals in general are widely studied and their vibrational spectra are well characterized. However, no specific review concerning alunite and jarosite spectroscopy and crystal structure has been forthcoming. This review focuses on the controversial aspects of the crystal structure and vibrational spectroscopy of jarosite and alunite minerals. Inconsistencies regarding band assignments especially in the 1000-400 cm(-1) region plague these two mineral groups and result in different band assignments among the various spectroscopic studies. There are significant crystallographic and magnetic structure ambiguities with regards to ammonium and hydronium end-members, namely, the geometry these two ions assume in the structure and the fact that hydronium jarosite is a spin glass. It was also found that the synthetic causes for the super cell in plumbojarosite, minamiite, huangite, and walthierite are not known. © 2013, Mineralogical Society of America.
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    Location of atoms in hydronium and ammoniojarosite
    (Australian Institute of Nuclear Science and Engineering, 2012-11-09) Spratt, HJ; Rintoul, L; Avdeev, M; Martens, WN
    The jarosite group of minerals have the general formula AB3(SO4)2(OH)6 and belong to the alunite supergroup. For jarosites the B site is iron (F3+) and for alunites the B site is aluminium (Al3+) Hydronium and ammoniojarosites have the hydronium (H30+) and ammonium (NH4+) cations at the A site. The crystal structure of most jarosites is space group R-3m (166), Z = 3 with the A site possessing D3d symmetry. However, H30+ and NH4++, are C3V and Td ions respectively and as such, have no inversion centre, yet the oxygen and nitrogen atoms are commonly located at an inversion centre, the D3d site. Wills and Harrison refined the hydronium oxygen at a lower symmetry C3V site, but no justification was given. To date, the NH4+ and H3Oh hydrogen atoms remain elusive and the geometry of these ions has not been determined. In addition, the possibility of a lower symmetry space group has not been investigated. A recent neutron powder diffraction study of hydronium and ammoniojarosites and also the corresponding alunites at room and low temperatures aims to resolve the H30+. and NH4+ structure and geometry.
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    Location of hydrogen atoms in hydronium jarosite
    (Springer Link, 2014-03-13) Spratt, HJ; Avdeev, M; Pfrunder, MC; McMurtrie, J; Rintoul, L; Martens, WN
    Various models for the crystal structure of hydronium jarosite were determined from Rietveld refinements against neutron powder diffraction patterns collected at ambient temperature and also single-crystal X-ray diffraction data. The possibility of a lower symmetry space group for hydronium jarosite that has been suggested by the literature was investigated. It was found the space group is best described as R3 ¯ m R3¯m, the same for other jarosite minerals. The hydronium oxygen atom was found to occupy the 3 ¯ ¯ ¯ m 3¯m site (3a Wyckoff site). Inadequately refined hydronium bond angles and bond distances without the use of restraints are due to thermal motion and disorder of the hydronium hydrogen atoms across numerous orientations. However, the acquired data do not permit a precise determination of these orientations; the main feature up/down disorder of hydronium is clear. Thus, the highest symmetry model with the least disorder necessary to explain all data was chosen: The hydronium hydrogen atoms were modeled to occupy an m (18 h Wyckoff site) with 50 % fractional occupancy, leading to disorder across two orientations. A rigid body description of the hydronium ion rotated by 60° with H–O–H bond angles of 112° and O–H distances of 0.96 Å was optimal. This rigid body refinement suggests that hydrogen bonds between hydronium hydrogen atoms and basal sulfate oxygen atoms are not predominant. Instead, hydrogen bonds are formed between hydronium hydrogen atoms and hydroxyl oxygen atoms. The structure of hydronium alunite is expected to be similar given that alunite supergroup minerals are isostructural.© 2014, Springer-Verlag Berlin Heidelberg.
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    The thermal decomposition of hydronium jarosite and ammoniojarosite
    (Springer Link, 2013-05-16) Spratt, HJ; Rintoul, L; Avdeev, M; Martens, WN
    The thermal decomposition of hydronium jarosite and ammoniojarosite was studied using thermogravimetric analysis and mass spectrometry, in situ synchrotron X-ray diffraction and infrared emission spectroscopy. There was no evidence for the simultaneous loss of water and sulfur dioxide during the desulfonation stage as has previously been reported for hydronium jarosite. Conversely, all hydrogen atoms are lost during the dehydration and dehydroxylation stage from 270 to 400 °C and no water, hydroxyl groups or hydronium ions persist after 400 °C. The same can be said for ammoniojarosite. The first mass loss step during the decomposition of hydronium jarosite has been assigned to the loss of the hydronium ion via protonation of the surrounding hydroxyl groups to evolve two water molecules. For ammoniojarosite, this step corresponds to the protonation of a hydroxyl group by ammonium, so that ammonia and water are liberated simultaneously. Iron(II) sulfate was identified as a possible intermediate during the decomposition of ammoniojarosite (421–521 °C) due to a redox reaction between iron(III) and the liberated ammonia during decomposition. Iron(II) ions were also confirmed with the 1,10-phenanthroline test. Iron(III) sulfate and other commonly suggested intermediates for hydronium and ammoniojarosite decomposition are not major crystalline phases; if they are formed, then they most likely exist as an amorphous phase or a different low temperature phases than usual. © 2013, Akadémiai Kiadó, Budapest, Hungary.