Mineralogy and Mass Specific Magnetic Susceptibility of Synthetic Aluminum Substituted Maghemites (γ-Fe2-xAlxO3).

Mineralogy and Mass Specific Magnetic Susceptibility of Synthetic Aluminum Substituted Maghemites (γ-Fe_2-xAl_xO₃)

M.A. Batista, A.C.S. Costa, I.G. Souza Junior & J.M. Bigham

Maghemite (γ-Fe₂O₃) is a ferrimagnetic iron oxide commonly found in the red, basalt-derived soils of Brazil.  Variable degrees of ionic substitution of Fe³⁺ by different cations (mainly Al³⁺) in the maghemite structure under natural conditions can influence many of the crystallochemical properties of this mineral.  To study these effects, in a systematic manner, synthetic Al-substituted maghemites were prepared by co-precipitation of FeSO₄^.7H₂O with increasing amounts of Al₂(SO₄)₃^.18H₂O in aqueous media.  The resulting magnetites were heated to 250ºC for 4 hours to form a series of brown maghemites that were subsequently characterized by chemical analysis, X-ray diffraction  specific surface area, and mass specific magnetic susceptibility.  Although Al - for - Fe substitution rates from 0.000 to 0.3000 mol mol^-1 were expected, the observed levels of solid solution ranged from 0.000 to 0.064 mol mol^-1, thereby forming a series of maghemites with compositions ranging from Fe_2.000Al_0.000O₃ to Fe_1.874Al_0.126O₃. An increase in Al - for - Fe substitution, [Al mol mol^-1] decreased the a₀ dimension of the cubic unit cells according to the regression equation: a₀ = 0.8357 - 0.0855Al (R² = 0.75).  A corresponding decrease was observed for the mean crystallite dimensions calculated using the Scherrer equation.  Mass specific magnetic susceptibility for the samples (χ_LF) initially increased with increasing Al-for-Fe substitution. The maximum value of χ_LF (52,503 10⁸ m³ kg^-1) was observed with 0.0246 mol mol^-1 of Al-substitution. After this point, the values of χ_LF decreased to 35,487 10⁸ m³ kg^-1. Frequency dependence values of the mass specific magnetic susceptibility (χ_FD) varied from 0.2 to 4.9%.  Maghemites with 0.021 to 0.064 mol mol^-1 Al (χ_FD ≥ 2) appear to be admixtures of superparamagnetic and coarser, single-domain and multi-domain grains.