Clays and Clay Minerals, Vol. 54, No. 2, 176-194, 2006.
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Authors
Wells, M.A.
Fitzpatrick, R.W.
Gilkes, R.J.
Issue Date
2006
Type
Article
Language
en_US
Keywords
Periodicals , Geology , Dehydroxylation , Differential Thermal Analysis , Double Endotherm , Al-, Cr-, Mn-, Ni- and Ti-substituted Goethite and Hematite , Domain Recrystallization
Alternative Title
Thermal and Mineral Properties Of Al-, Cr-, Mn-, Ni- And Ti-Substituted Goethite
Abstract
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 O2ÿ ions for OHÿ (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 OHÿ 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.
Description
gsccm-54-2-04-wel.pdf 1.41MB PDF/A Document.
Citation
Clays and Clay Minerals, Vol. 54, No. 2, 176-194, 2006.
Publisher
The Clay Minerals Society
License
Copyright © 2006-2018, The Clay Minerals Society