Abstract
A comparison between a microscopic observation and magnetic examinations such as thermomagnetic curve, low-temperature characteristics and microscopic coercivity spectrum indicates that the magnetic stability of granites is primarily controlled by the grain size: stable granites abundantly possess extremely fine magnetite grains (grain size<5μ), whereas the unstable sample contains mostly coarse magnetite grain (grain size ≥50μ).
Magnetic characteristics of VRM of magnetite grain which is produced at relatively high temperatures are examined. VRM produced at higher temperature generally becomes magnetically harder. The increase rate of the magnetic hardness which is expressed as a recovery ratio in low-temperature demagnetization is maximum at about 200°C. However, the extrapolation of the experimental results to a geological condition indicates the recovery ratio (or the magnetic hardness) of VRM produced in a geological condition will be only several times that of IRM produced at room temperature. Therefore, the VRM of granites which must have been produced over a long period of time at relatively high temperatures may still be effectively erased by low-temperature demagnetization.
