Grain‐size dependence of the rock magnetic properties for a natural maghemite

The natural maghemite from the Robe River mining district (Australia) is shown to be thermally very stable. The maghemite/hematite inversion occurs at ∼650°C for sized fractions (250 µm to < 5 µm), enabling determination of Tc for maghemite at 610°C. The maghemite is intimately intergrown with hematite and Js of the maghemite (corrected for the hematite content) is 64 Am²/kg. Jrs, Hcr and Hc range between 7.46–10.18 Am²/kg, 16.90–23.30 mT, and 6.38–9.00 mT respectively, indicating PSD maghemite with grain sizes between ∼1 and ∼20 µm. The grain‐size dependence of the rock magnetic parameters of the maghemite is less prominent than those of magnetite of nominally the same size. It is suggested that the LT‐behavior of ARM can be diagnostic of the presence of maghemite: upon cooling to −196°C it shows no change; upon rewarming a gradual decrease in intensity occurs starting at ∼ −120°C.