Superparamagnetism in Red Sandstones

Abstract

Susceptibility measurements down to liquid hydrogen temperatures have been made on rocks with (a) unstable and (b) stable natural remanent magnetization. The former follow a Curie Law and this indicates that most of the magnetic content is apparently paramagnetic. Measurements in different fields fail to reveal the weak ferromagnetism responsible for the natural remanence. The latter measurements suggest that an apparent paramagnetic component is present together with a component having a susceptibility independent of temperature. In both cases the magnetization-applied field relationship at liquid helium temperature in fields up to 20000 oersteds is not linear and this probably indicates that both rocks contain ferromagnetic material in a superparamagnetic state, the particle size of which is calculated to be of the order of 20 Å. The material is thought to be hydrous alpha ferric oxide of which the cement is made. Viscous magnetization of about the same strength as the weak fossil remanence has previously been observed in the unstable rock but is not noticeable in low fields in the stable one. The remanent magnetization of red rocks is thought to be due in part to grains of the red cement larger than those responsible for viscous magnetization and Superparamagnetism and in part to black haematite grains. These are sparsely distributed in the unstable rock but are more abundant in the stable one. Magnetization of the cement might occur soon after formation, or during a sintering process during burial at depth, or in some cases in geologically recent times due to tropical weathering. The calculated grain size of about 20 Å for the superparamagnetic particles differs appreciably from that of about 1000 Å previously estimated for the viscous particles. Experiments are being made on artificially produced haematite of known particle size to attempt to resolve this anomaly.