Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Wells, M. A. Articles by Gilkes, R. J. Search for Related Content GeoRef GeoRef Citation

THERMAL AND MINERAL PROPERTIES OF Al-, Cr-, Mn-, Ni- AND Ti-SUBSTITUTED GOETHITE

¹ CSIRO Exploration and Mining, Australian Resources Research Centre (ARRC), PO Box 1130, Bentley, WA 6102, Australia
² CSIRO Land and Water, The University of Adelaide, Glen Osmond, Adelaide, SA, 5064, Australia
³ School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA, 6009, Australia

^* E-mail address of corresponding author: Martin.Wells{at}csiro.au

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 Al³⁺, Cr³⁺, Mn³⁺, Ni²⁺ and Ti⁴⁺ 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^2– 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.

Key Words: Dehydroxylation • Differential Thermal Analysis • Double Endotherm • Al-, Cr-, Mn-, Ni- and Ti-substituted Goethite and Hematite • Domain Recrystallization

This article has been cited by other articles:

				N. Taitel-Goldman, V. Ezersky, and D. Mogilyanski HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY STUDY OF Fe-Mn OXIDES IN THE HYDROTHERMAL SEDIMENTS OF THE RED SEA DEEPS SYSTEM Clays and Clay Minerals, August 1, 2009; 57(4): 465 - 475. [Abstract] [Full Text] [PDF]