Dehydroxylation of Mn goethite: a structural study

MARIANA ALVAREZ; ELSA E. SILEO; ELSA H. RUEDA

Foz de Iguazu

Conferencia; 11th International Colloid and Surface Conference; 2003

International Association of Colloid and Interface Scientists

Dehydroxylation of goethites in soil with the formation of hematite may occur in some environments due to natural and managed fires. Most research on the goethite dehydroxylation process has focused on non-substituted goethite or Al-substituted goethite. However, the replacement of Fe(III) by other metals in the goethite structure may alter the decomposition process, in fact our previous results[1] have shown that the temperature for the transformation is raised when Fe(III) is substituted by Mn(III) and this effect increases with increasing Mn substitution in these minerals. In this work we explore the thermal transformation of Mn-goethite to Mn-hematite, as a part of a general study of how manganese substitution and dehydroxylation influence the mineralogical and surface chemical properties of some Fe oxides. The temperature range studied was 180-600 ºC and nominal molar amounts of Mn in the sample were kept to 0-10 % to insure goethite as an only starting phase. The degree of transformation was followed by the ratio: hematite/(goethite + hematite) calculated from the intensities of XRD diffraction lines. The formation of hydrohematite (Fe2-x/3(OH)x(OH)3-x) was also investigated through the Rietveld refinement of selected samples. Scanning electron microscopy (SEM), infrared spectroscopy (IR) and differential thermal analysis (DTA) techniques were also applied to characterize intermediate and final products. The results obtained in this work were compared with those of Al-goethite submitted to similar thermal treatment.       [1] M. Alvarez, E. Sileo and E. Rueda (2001) Acid dissolution of synthetic manganese goethite before and after transformation to hematite by heating. The 12th International Clay Conference, Bahía Blanca, Argentina. H. Ruan and R.J. Gilkes (1995) Dehydroxilation of aluminous goethite: unit cell dimensions, crystal size and surface area. Clays and Clay Minerals, 43, 196-211.