Mineralogical, Chemical, and Physical Characterization of Synthetic Al-Substituted Maghemites (γ−Fe₂−_xAl_xO₃)

Abstract

Maghemite (γ-Fe2O3) is a ferrimagnetic Fe oxide commonly found in tropical and subtropical soils, especially in the topsoil where it is usually a product of burning. Isomorphic substitution (IS) of the Fe in maghemite by different metals (mainly Al3+) can modify its mineralogical and chemical attributes, and these modifications may be important to understanding the formation and properties of this mineral in soils and sediments. The objective of this work was to evaluate the crystallochemical alterations of synthetic, Al-substituted maghemites prepared by the precipitation of magnetites from alkaline aqueous media containing FeSO4·7H2O with increasing amounts of Al2(SO4)3·7H2O to obtain hypothetical Al3+ for Fe3+ substitutions ranging from 0.0 to 40.0 mol %. The Al-substituted magnetites were washed and dried, and then heated to 250ºC for 4 h to form yellowish red maghemites that were characterized by total chemical analysis, X-ray diffraction, specific surface area (SSA), mass-specific magnetic susceptibility, infrared spectroscopy, transmission electronic microscopy, and color. Increasing Al3+ substitution to an experimental maximum of 15.9 mol % decreased both the a0 dimension of the cubic unit cell (a0 = 0.8339 − 396.157 × 10−16 Al, r2 = 0.99) and the mean crystallite dimension (MCD = 76.4–3.15Al, r2 = 0.79) of the maghemites. With the decrease in MCD came a more yellowish color, an increase in SSA, and a decrease in crystallinity as measured through extraction of the samples with acid ammonium oxalate. The mass-specific magnetic susceptibility of the maghemites increased with Al3+ substitution up to 5.3 mol % and then decreased with further replacement of Fe by Al. Solid-phase aluminum in excess of 16 mol % substitution appeared to occur as a separate, poorly crystalline phase that was X-ray amorphous.

Thermal Transformation and Characterization of Synthetic Al-Substituted Maghemites (γ-Fe2-xAlxO3)

“…Therefore, the temperature of change in soil colors is dependent on the mineralogy. Figure 4); i.e., our values are higher than those registered by Dearing (1994) and similar to those obtained by Batista et al (2010; for synthetic maghemites. With increasing IS, χ LF values decreased (Figure 4b) since Al 3+ is a paramagnetic ion (Cullity, 1972;Wolska, 1990) and has a similar dilution effect on magnetization, as observed previously for Zn (Batista et al, 2008).…”

“…Changes in the color of Fe oxide because of IS of Fe 3+ by other metallic cations have been observed by some researchers (Scheinost et al, 1999;Batista et al, 2010). The color of Al-substituted maghemite (γ-Fe 2-x Al x O 3 ) observed by Batista et al (2010) ranged from 4.8YR to 7.1YR.…”

“…The maghemite hue values (Figure 2) tend to increase with increasing isomorphic substitution (IS) but without any significant correlation (p>0.05 The Fe-to-Al isomorphic substitution effect on mineral color is difficult to detect because Al substitution reduces particle size and changes chemical composition, which might also change mineral color (Batista et al, 2010). Maghemite and hematite hues ranged from 5.1YR to 7.1YR, and from 0.3YR to 2.9YR, respectively.…”

“…Aluminous maghemites were prepared by the oxidation of Al-magnetites, synthesized by co-precipitation in alkaline KNO 3 solutions containing FeSO 4 .7H 2 O with stoichiometric amounts of Al 2 (SO 4 ) 3 .7H 2 O, according to the procedure described by Schwertmann and Cornell (1991) and Batista et al (2010). Magnetite synthesis was performed in a N 2 atmosphere over 2 h periods to assess the impact of equilibration time on Al substitution.…”

“…The color of Al-substituted maghemite (γ-Fe 2-x Al x O 3 ) observed by Batista et al (2010) ranged from 4.8YR to 7.1YR. Torrent and Barrón (2003) observed that the color of synthetic and natural Al-substituted hematite ranged from 9.5P to 5.3YR.…”