Problems in interpreting AMS parameters in diamagnetic rocks
The AMS parameters widely used to characterize the magnetic fabric of rocks (L, F, P, T) are not directly applicable to diamagnetic rocks. Namely, there are two principal methods of their definition: (1) from the signed principal susceptibilities or (2) from the absolute values of the principal susceptibilities. The first method results in L, F and P values less than one in contrast to materials whose susceptibility arises from paramagnetism and/or ferromagnetism sensu lato, where they are greater than one by definition. The ratio parameters decrease with increasing intensity of alignment of the magnetic minerals; the calculated shape parameters are, however, correct. The second method gives L, F and P values greater than one; they increase with increasing preferred orientation, however the ellipsoid shape parameters are inverse. The development of AMS parameters with increasing preferred orientation is modelled mathematically for quartzite/evaporite (para- and/or ferromagnetic AMS superimposed on the isotropic diamagnetic matrix) and for marble/limestone (preferred orientation of calcite in a rock). A rational set of AMS parameters is then recommended.