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 similar to 650 degrees C for sized fractions (250 mu m Co < 5 mu m), enabling determination of T-c for maghemite at 610 degrees C. The maghemite is intimately intergrown with hematite and J(s) of the maghemite (corrected for the hematite content) is 64 Am-2/kg. J(rs), H-cr and H-c range between 7.46 - 10.18 Am-2/kg, 16.90 - 23.30 mT, and 6.38 - 9.00 mT respectively, indicating PSD maghemite with grain sizes between similar to 1 and similar to 20 mu m. 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 degrees C it shows no change; upon rewarming a gradual decrease in intensity occurs starting at similar to -120 degrees C.