Frequency-dependent susceptibility measurements of environmental materials

The theory, measurement and interpretation of frequency-dependent susceptibility (χFD) are examined. A new model is proposed which explains χFD in terms of the behaviour of all superparamagnetic grains (SP) with diameters between 0 and ≈0.03 μm. The model predicts maximum χFD percentage values of 14–17 per cent for spherical SP ferrimagnetic grains in the grain-size range 0.01–0.025 μm, and a maximum value of 10–12 per cent for grain assemblages spanning a wider range of grain sizes (0–0.03 μm). Synthetic and experimental data support the model predictions in terms of both maximum χFD percentage values and the relationship between χFD percentage and mass specific χFD, which exhibits an envelope of data points partly related to grain-size distributions within the SP range. When the χFD percentage is at a maximum, the mass specific χFD term can be used to estimate the concentration of SP grains in a sample. Lower values of χFD percentage in soils are caused by the presence of narrow distributions of ultrafine SP grains, frequency-independent stable single and multi-domain ferrimagnetic grains. Some soils with low susceptibilities may have low χFD percentages because of an appreciable content of paramagnetic and canted antiferromagnetic minerals. A simple mixing model predicts proportions of SP grains in mixed grain assemblages, but model validation requiring further characterization of grain interaction and grain-size distributions is needed before it can be applied to environmental data.