ABSTRACT Maghemite (γ-Fe2O3) and hematite (α-Fe2O3) are important iron oxides in Brazilian soils derived from basalt. Maghemite can transform into hematite when exposed to high temperatures. However, isomorphic substitution (e.g., Al3+) may largely influence this process. We analyzed the kinetics of thermal transformation of Al-maghemites into Al-hematites and some of its mineralogical aspects. Synthetic substituted maghemites with different degrees of Al-substitution (0.0, 1.0, 2.0, 2.9, 3.8, 5.6, 6.7, 10.0, 12.0, and 17.1 mol% Al) were subjected to a temperature of 500±10 °C for 0, 5, 10, 16, 64, 128, 192, 360, 720, 2160, 3600, 5040, and 6480 min. After thermal treatment, samples were characterized by X ray diffraction (XRD), differential thermal analysis (DTA), specific surface area (SSA) measurement, and total chemical analysis. XRD patterns were analyzed by Rietveld refinement, and maghemite and hematite contents were calculated using Rietveld refinement and the method proposed by Sidhu. Isomorphic substitution of Fe by Al increased the critical temperature of transformation and the time necessary for maghemite to hematite transformation. Rietveld refinement data showed a better fit than the data adjusted by the Sidhu method. Increasing isomorphic substitution also decreased lattice parameters and mean crystallite dimension (MCD) values in maghemite; but only c-dimension and MCD decreased with increasing Al-substitution in hematite. For maghemite, the SSA increased with isomorphic substitution, rising up to 5.9 mol% Al; for hematite, SSA increased linearly. SSA decreased with heating time, regardless of isomorphic substitution.

ABSTRACT Burning is a common practice in tropical areas and related changes in mineralogy might affect the chemical and physical behavior of soils. Maghemite is a common iron oxide in soils formed from basic rocks in tropical regions. This mineral and hematite are the main pigments in these soils and exhibit high magnetization stemming from the precursor magnetite formed during the weathering process of primary minerals. The objective of the present study was to analyze changes in color, magnetic suceptibility values, Fourier transform infrared spectroscopy (FTIR) spectra, and available Fe and Al contents extracted with 1 mol L-1 KCl during the process of thermal transformation of synthetic Al-maghemites into Al-hematites. Synthetic substituted maghemites with different degrees of Al-substitution (0.0, 1.0, 2.0, 2.9, 3.8, 5.6, 6.7, 10.0, 12.0, and 17.1 mol% Al) were subjected to a temperature of 500 ± 10 °C for 0, 5, 10, 16, 64, 128, 192, 360, 720, 2160, 3600, 5040 and 6480 min. The color of the samples was analyzed by a Munsell system in a colorimeter. Mass-specific magnetic susceptibility (χLF) was measured at low-frequency. Available Fe and Al contents were estimated by a 1 mol L-1 KCl solution. Fourier transform infrared spectroscopy (FTIR) spectra were obtained through use of a Bruker Vertex 70X FTIR spectrophotometer at a spectral resolution of 4 cm-1. Contents of Fe and Al extracted by 1 mol L-1 KCl in Al-hematites were not detected. All samples analyzed exhibited YR hue. Hue proportion decreased with increased heating time, and color changed from brown to red. The increase in isomorphic substitution (IS) led to increased hue values from maghemite to hematite, and the latter then became yellower. The χLF values decreased with an increase in heating time, indicating transformation from a ferrimagnetic phase (maghemite) to an antiferrimagnetic phase (hematite). With increasing IS, the maghemite χLF values decreased. Bands of the initial members (time 0) in the FTIR spectra were indexed as maghemites. The end members after completion of the heat treatment were identified as hematites. The IS of Fe by Al in maghemite influenced the thermal transformation to hematite, as well as the color and χLF of the minerals. The χLF proved to be very efficient in detecting maghemites remaining after thermal processing. Fe and/or Al were not ejected from the hematite crystalline structure after heat treatment.

Maghemitas sintéticas substituídas com Al foram caracterizadas por meio de análise química total, difratometria de raios-X (DRX), espectroscopia Mössbauer (EM) e magnetometria de amostra vibrante (MAV). Objetivou-se com este trabalho determinar as propriedades estruturais, magnéticas e hiperfinas de γ-Fe2-xAl xO3, conforme a variação da concentração de Al.Os resultados de DRX dos produtos sintéticos foram indexados somente para maghemita. Com o aumento da substituição de Fe por Al, o diâmetro médio do cristalito diminuiu e levou todos os picos de difração para maiores ângulos º2θ. A dimensão a0 da cela unitária cúbica decresceu com o aumento de Al, de acordo com a equação a o = 0,8385 - 3,63 x 10-5 Al (R²= 0,94). A maioria dos espectros Mössbauer foi composta de um sexteto, mas na taxa mais alta de substituição de 142,5 mmol mol-1 Al um doubleto e sexteto foram obtidos a 300 K. Todos os parâmetros hiperfinos do subespectro foram consistentes com o alto spin Fe3+ (0,2 a 0,7 mms-1) e sugeriram um forte componente superparamagnético associado com o dubleto. O campo magnético hiperfino dos sextetos decresceram com a substituição de Al [Bhf (T) = 49,751 -0,1202Al; R² = 0,94], enquanto as larguras da linha aumentaram de forma linear. A magnetização de saturação também diminuiu com o aumento da substituição isomórfica.

Synthetic aluminum-substituted maghemites were characterized by total chemical analysis, powder X-ray diffraction (XRD), Mössbauer spectroscopy (ME), and vibrating sample magnetometry (VSM). The aim was to determine the structural, magnetic, and hyperfine properties of γ-Fe2-xAl xO3 as the Al concentration is varied. The XRD results of the synthetic products were indexed exclusively as maghemite. Increasing Al for Fe substitution decreased the mean crystalline dimension and shifted all diffraction peaks to higher º2θ angles. The a0 dimension of the cubic unit cell decreased with increasing Al according to the equation a o = 0.8385 - 3.63 x 10-5 Al (R²= 0.94). Most Mössbauer spectra were composed of one sextet, but at the highest substitution rate of 142.5 mmol mol-1 Al, both a doublet and sextet were obtained at 300 K. All hyperfine parameters from the sub-spectra were consistent with high-spin Fe3+ (0.2 a 0.7 mms-1) and suggested a strong superparamagnetic component associated with the doublet. The magnetic hyperfine field of the sextets decreased with the amount of Al-substitution [Bhf (T) = 49.751 - 0.1202Al; R² = 0.94] while the linewidth increased linearly. The saturation magnetization also decreased with increasing isomorphous substitution.