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SUMMARY Iron and sulphur in proportions appropriate to Fe 7 S 8 were reacted in evacuated quartz tubes for 24 hr at 500°C. Quenching produces predominantly hexagonal pyrrhotite, whereas further prolonged annealing at 250°C produces monoclinic pyrrhotite. We mainly report the properties of material cooled from 500° Ca t 10°C min'1, which is predominantly monoclinic but probably contains hexagonal pyrrhotite together with pyrite, FeS 2 . This material, which we refer to as 'non-ideal' pyrrhotite, may be a better analogue of pyrrhotite ore than ideal synthetic monoclinic pyrrhotite. The samples were characterized by X-ray, Mossbauer eVect and thermomagnetic analyses. The slow- cooled non-ideal monoclinic pyrrhotite exhibits thermal stability intermediate between monoclinic and hexagonal pyrrhotites. One striking observation is that the deviation from ideal monoclinic pyrrhotite increases with decreasing particle size. Particles about 1 mm in size have a low saturation magnetization value of about 6 Am2 kg'1, indicating a lower concentration of ferrimagnetic monoclinic pyrrhotite (saturation magnetization 18 Am2 kg'1) than particles of about 6 mm and above (saturation magnetization 12 Am2 kg'1). On the other hand, magnetization process parameters—coercive force, ratio of saturation remanence to saturation magnetization, coercive force of remanence, median destructive and inductive fields, the magnetic susceptibility—follow well-behaved power law dependences on particle size, similar to monoclinic pyrrhotite. Magnetic domain patterns in the slow-cooled non-ideal pyrrhotite are similar in type and style to those observed in synthetic monoclinic pyrrhotite and in natural pyrrhotite particles in rocks. Dynamic observations of domain walls in changing external alternating and direct magnetic fields are consistent with macroscopic obser- vations of magnetization change with field. The critical size for the monodomain/ multidomain transition in synthetic monoclinic pyrrhotite, at about 1 mm, is essentially the same as that previously determined for natural pyrrhotite grains in rocks. The observations suggest that the domain walls lie normal to the c-axis of the structure, consistent with the known magnetocrystalline anisotropy symmetry of monoclinic pyrrhotite. The type and style of the magnetic microstructures of hexagonal pyrrhotite are quite diVerent from those of monoclinic pyrrhotite. The domain patterns are on a much smaller scale, wavy, and sometimes resemble those of a hexagonal material such as magnetoplumbite or cobalt.

SUMMARY Palaeomagnetic investigations have been carried out on Tethyan sediments from the NW Zanskar Range (33.9°N, 76.5°E). A total of 455 oriented samples from 41 sites were drilled from four stratigraphic units of Middle Cretaceous to Early Eocene age (Shillakong Formation, Marpo Limestone/Spanboth Formation, Stumpata Quartzarenite/Spanboth Formation, Kong Formation). A characteristic remanence (ChRM) could be isolated for 27 sites (254 specimens) through detailed thermal and alternating field demagnetization and multicomponent analysis. All the ChRM directions identified are similar, irrespective of geological age. They fail the fold test and yield an overall in situ mean direction of D= 336.6°/I=+39.6° with α95= 5.9° and k= 22.9. This corresponds to a palaeopole position at 66.5°N, 323.5°E (N-pole). Isothermal remanence (IRM) acquisition and thermal demagnetization of a saturation IRM identify pyrrhotite as the dominant ferrimagnetic mineral and carrier of the ChRM. The pyrrhotite remanence is probably a thermoremanent magnetization acquired when the temperature of low-grade metamorphism decreased below about 300°C. The ChRM directions are thus clearly secondary in origin, and a negative conglomerate test carried out on the Kong Formation demonstrates that remanence acquisition post-dates the Early Eocene. The mean ChRM inclination suggests that the remanence was acquired at about 20°N. Cooling events in the higher Himalayan crystallines indicate an Oligocene age for this remanence acquisition. The mean ChRM declination shows a counterclockwise rotation of 23.4° since remanence acquisition. Assuming remanence acquisition at about 30 Myr ago, a slight counterclockwise rotation of about 10° relative to the stable Indian Plate can be concluded. This does not fit with the general pattern of palaeomagnetic results from neighbouring areas within the western syntaxis of the Himalaya, from which a clockwise rotation relative to the stable Indian Plate is expected for the Zanskar Range.