Magnetic Mineralogy

Magnetite, maghemite, and hematite comprise the magnetic mineralogy of loess and paleosol. Fine et al. [1993; in press] estimated that 70% of the total iron in the loess column is in detrital silicate minerals and that most of the remaining Fe resides in hematite. Hematite is unimportant for the magnetic record of climate change, however, having negligible magnetic susceptibility relative to the ferrimagnetic minerals. Less than about 5% of the total Fe is tied up in detrital and pedogenic ferrimagnetic grains, with most of this amount in CBD-soluble phases [ Fine et al., in press]. Many authors identify multidomain titanomagnetite or cation-deficient magnetite as the principal detrital ferrimagnetic mineral [ Maher and Thompson, 1992; Rolph et al., 1993; Banerjee et al., 1993; Xu et al., 1991; Fine et al., 1993; Verosub et al., 1993]. Disagreement exists, however, about the mineral type of the ultrafine pedogenic magnetic grains.

The dominance of magnetite in paleosols is indicated, primarily using thermomagnetic techniques, by Heller et al. [1991]; Liu et al. [1991, 1992]; Rolph et al. [1993]; and Maher and Thompson [1991, 1992]. In contrast, a significant contribution from maghemite is interpreted on the basis of X-ray diffraction [ Xu et al., 1991], CBD treatment [ Verosub et al., 1993], and Mössbauer spectroscopy [ Vandenberghe et al., 1992]. The interpretations regarding pedogenic magnetic phases are complicated by a number of factors, including the possible ineffectiveness of the separation techniques to isolate all important magnetic minerals for some mineralogic tests; the possible post-depositional alteration of magnetite to produce maghemite; and an incomplete understanding of the effects of CBD treatment on different minerals [see Maher et al., 1994a; Hunt et al., in press].

Eyre and Shaw [1994] interpreted thermal demagnetization of low-temperature isothermal remanent magnetization and thermomagnetic curves to indicate a dominance of maghemite in paleosols. They suggested that size-induced phase transitions in FeO energetically favors maghemite for certain ultrafine grain sizes between 100 and 300 Angstroms. If so, a population of maghemite grains in this size range would possess both SP and SD behavior.