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Feldspars are the most abundant rock-forming minerals in the Earth’s crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10−9 m3 kg−1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

Decoupling of paramagnetic and ferrimagnetic AMS development during the experimental chemical compaction of illite shale powder

SUMMARY In recent years remagnetization of orogenic belts has been explained by fluid migration through rocks undergoing deformation. A laboratory study of remagnetization is presented in which varying amounts of iron (0-13.5 weight per cent Fe,O,) are adsorbed onto smectite surfaces. All smectite samples contain structural Fe (111) which is located in octahedral sites and is thermally stable up to 700°C. An increase in the amount of iron adsorbed onto the clay surface leads to the formation of ferric nanophases in which parts are magnetic. Mineralogical changes that occur during thermal treatment between room temperature and 700 "C were monitored using electron spin resonance (ESR), bulk susceptibility, acquisition of isothermal remanent magnetization (IRM) and Curie temperature analysis. After heating the samples to 250"C, a new ferrimagnetic phase is created as indicated by ESR and IRM acquisition. ESR spectra, IRM acquisition and Curie analyses suggest that magnetite is the predominant phase that is being created. These grains continue to be created and grow with heating up to 500°C. Above this temperature a decrease in the intensity of the IRM at 1T suggests that the phase is being transformed into haematite. The thermal experiments on iron-loaded smectites show that surfaceinduced processes can lead to the formation of new magnetic minerals under conditions characteristic of low-grade metamorphism.