Oriented Transformations in Iron Oxides and Hydroxides

Hydrothermal Formation of Ferrite Spinels

“…In no case was the rehydration of hematite to goethite observed ; as Langmuir (6) pointed out, this is thermodynamically possible below 80°, or above this temperature for hematite of very small grain size, but it is unlikely to be observed for kinetic reasons (however, in soils, hematite may be dissolved by complexing and/or reduction by organic anions followed by reprecipitation as goethite (21,22)). The temperature range found in the present studies for the goethite-hematite conversion agrees with that reported by several other investigators (23)(24)(25)(26) for the reaction both in water and in air (which supports our assertion that the reaction does not involve the solution phase), but lower decomposition temperatures have been recorded (1,(27)(28)(29)(30)(31)(32)(33)(34)(35). These lower decomposition temperatures could be associated with small particle size, the importance of which was stressed by Langmuir (6), o r with the presence of impurities, since we have found that Co(OH), and Mn(OH), accelerate the reaction significantly.…”

Annealing Effects on the Metal and Semiconductor Nanowires Loaded Inside the Alumina Pores

“…This observation is in line with the indication (Gehring & Hofmeister, 1994) that, on the molecular level, the removal of OH groups in the lepidocrocite begins at 142-155 ºC. Based on the results obtained here by Mössbauer transmission spectroscopy and Raman scattering (not shown here) it seems likely that the second step, corresponding to the dehydroxylation of iron oxyhydroxide nanowires to γ-and a-Fe 2 O 3 , completely proceeds in the vicinity of 500 ºC although from literature data (Bernal et al 1957) the rhombic structure of synthetic lepidocrocite reconstructs to cubic maghemite in the 230-250 ºC region which at 375 ºC is transported completely to hematite with a hexagonal structure, e.g. at much lower temperatures.…”

Origin, mineralogy, and chemistry of manganiferous and ferruginous precipitates found in sand and gravel deposits in Finland

“…Transformation to a-structure occurs totally at c. 400°C (Bernal et al 1957). Transformation of lepidocrocite to maghemite is pseudomorphic.…”

“…When lepidocrocite is heated, j'-Fe 2 0 3 (Rooksby 1972), maghemite (Bernal et al 1957), is the first mineral to form.…”