Goethite dehydroxylation

Pores may be present as structural features (e. g. between domains) or as a result of aggregation of particles. They may also be the result of partial dehydroxylation (oxide hydroxides) or dissolution. Although the shapes of pores can be quite variable, there are some definite, basic forms. The commonest of these are 1) slit shaped, the walls of which may or may not be parallel 2) ink bottle which are closed upon all sides but one from which a narrow neck opens and 3) cylindrical. Upon partial dissolution, pores bounded by well-defined crystal planes (e. g. 102 in goethite) develop (Chap. 12). 

Hematite formed by dehydroxylation of oxide hydroxides at temperatures below 500-600 °C is porous. That formed by heating goethite in vacuo at 300 °C contains slit shaped meso pores which coalesce to circular macropores at temperatures >400°C (Naono and Fujiwara, 1980). At even higher temperatures, these pores are... 

Under otherwise similar conditions, low oxidation rates appear to promote magnetite and goethite, whereas high rates favor lepidocrocite. Magnetite formation probably requires slow oxidation because complete dehydroxylation of the precursor (green rust) prior to complete oxidation is only possible if sufficient time is available if, on the other hand, complete oxidation is fast and precedes dehydroxylation, lepidocrocite forms in preference to magnetite (Schwertmann Taylor, 1977). Dehydroxylation and oxidation appear to be competing reaction steps. 

Goethite Hematite Thermal or mechanical dehydroxylation Hydrothermal dehydroxylation Gas/vacuum Solution... 

Dehydroxylation of goethite produces the ferrite reds - extremely colour fast and pure hematite. With low temperature calcination the acicular shape of the goethite precursor is retained, whereas high temperatures lead to a sintered product. 

Paterson, E. Swaffield, R. (1980) Influence of adsorbed anions on the dehydroxylation of synthetic goethite. J. Thermal Analysis 18 161-167... 

Wells, M.A. Gilkes, R.J. Anand, R.R. (1989) The formation of corundum and aluminous hematite by the thermal dehydroxylation of aluminous goethite. Clay Min. 24 513-530 Wells, M.A. Gilkes, R.J. Fitzpatrick, R.W. (2001) Properties and acid dissolution of metal-substituted hematites. Clays Clay Min. 49 60-72... 

Z. f Polymere 215 57-60 Wolska, E. Schwertmann, U. (1989) Nonstoi-chiometric structures during dehydroxylation of goethite. Z. Kristallogr. 189 223—237 Wolska, E. Schwertmann, U. (1989 a) Selective X-ray line broadening in the goethite-de-rived hematite phase. Phys. Stat. Sol. A 114 K11-K16... 

Thermal methods of analysis are often useful in the characterization of minerals, as described in Section 7.6.5. Aluminum hydroxides such as gibbsite show a mass loss of 34.6% on dehydroxylation thus, they show an important negative peak in DTA and a marked mass loss in TGA, and so these techniques are employed for both qualitative and quantitative characterization of these minerals. The same happens with iron hydroxides and oxohydroxides, such as goethite, lepidocrocite, and so on also, the presence of OH groups in otherwise thermally inert minerals such as hematite can be detected. 

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