High-field remanence properties of synthetic and natural submicrometre haematites and goethites: significance for environmental contexts.

Haematite and goethite are significant magnetic components both of marine and terrestrial sediments. Variable magnetic behaviour in haematite and goethite has been reported, reflecting variations in grain size, crystal defects, nonstoichiometry and mode of formation. Here, we provide new data, first, for a range of synthetic haematite and goethite powders of known grain size and, second, for a variety of haematite- and goethite-bearing natural samples, including red bed samples and modern and fossil soils. Based on the synthetic data, we identify two high-field parameters which may be of value in identifying and characterizing the high-coercivity components of natural environmental samples (even when these are dominated magnetically by the presence of trace concentrations of ferrimagnets). H% indicates the proportion of the room temperature (RT) remanence acquired in fields from 2 Tesla (T) up to 7 T and Hcool% indicates the increase or decrease in high-field remanence upon cooling in zero field to 77 K (LT). Used in tandem, these parameters can differentiate between goethite-dominated, and haematite-dominated samples, and indicate differences in haematite grain size. H% values for the synthetic haematites vary between 27% and 38%, with some apparent grain size-dependence, and for the goethites, between 84% and 92%. Hcool% values range from +130% to +157% for the goethites and �75% to �95% for those haematites of grain size >100 nm