Thermal and mineral properties of A1-, Cr-, Mn-, Ni- and Ti-substituted goethite
Wells, M. A. (CSIRO Exploration and Mining. Australian Resources Research Centre)
Fitzpatrick, R. W. (The University of Adelaide)
Gilkes, R. J. (University of Western Australia. School of Earth and Geographical Sciences)

Data:	2006
Resum:	Mineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al, Cr, Mn, Ni and Ti in the goethite structure. A weight loss of 6. 2 wt. % for goethite containing 12. 2 mol. % Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O ions for OH (i. e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4. 5 mol. % Al, 15. 3 mol. % Mn and 12. 2 mol. % Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite ('high-a' vs. 'low-a') as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.
Drets:	Tots els drets reservats.
Llengua:	Anglès
Document:	Article ; recerca ; Versió publicada
Matèria:	Dehydroxylation ; Differential Thermal Analysis ; Double Endotherm ; Al- ; Cr- ; Mn- ; Niand Ti-substituted Goethite and Hematite ; Domain Recrystallization
Publicat a:	Clays and Clay Minerals, Vol. 54, Núm. 2 (2006) , p. 176-194, ISSN 1552-8367

DOI: 10.1346/CCMN.2006.0540204

19 p, 1.3 MB  Accés restringit a la UAB

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