Reflectivity (visible and near IR), Mössbauer, static magnetic, and X ray diffraction properties of aluminum‐substituted hematites

Abstract

Because aluminum substitution for iron occurs in polymorphs of Fe2O3 and FeOOH in terrestrial (and by inference Martian) environments, it is important for the mineralogical remote sensing of both planets at visible and near‐IR wavelengths to know the effects of Al substitution on their reflectivity spectra. Diffuse reflectivity (350–2200 nm), Mössbauer, static magnetic, and X ray diffraction data are reported for a series of aluminum‐substituted hematites α‐(Fe,Al)2O3 for compositions having values of Als. (mole ratio Al/(Al+Fe)) up to 0.61. Samples were prepared by oxidation of magnetite, dehydroxylation of goethite, and direct precipitation. Unit cell dimensions decrease with Als but at a rate less than that predicted by the Vegard rule. At 293 K, Mössbauer spectra are sextets (negative quadrupole splitting, QS) for Als up to ∼0.5 and doublets for larger Als. At 21 K, all compositions are sextets; however, there is a discontinuity of ∼0.5 T in the magnetic hyperfine field (Bhf) and QS changes sign at Als = 0.06(2) (Morin transition). Al‐poor compositions have positive QS and higher Bhf. Negative and positive quadrupole splittings are indicative of the weakly ferromagnetic and antiferromagnetic states of hematites, respectively. The position of the least energetic crystal‐field transition (6A1→4T1g) of ferric iron shifts to longer wavelengths with increasing Als. The magnitude of the shift is a linear function of (1/a)5, where a is the hexagonal unit cell dimension. For geologically reasonable amounts of Al substitution (Als < 0.33), the magnitude of the shift is small (∼20 nm), so that it is problematical (based on reflectivity data alone) to uniquely ascribe shifts in 4T1g band positions to different degrees of Al substitution. On the basis of Martian spectral data, the range in Als for Martian hematites is 0 < Als < 0.19.