Correlation between magnetic anisotropy and phyllosilicate preferred orientation for various sedimentary rocks.

The low and high-field magnetic anisotropy (AMS, HFA) of the various sedimentary and anchimetamorphic rocks was compared to the theoretical anisotropy calculated from the neutron texture goniometry measurements. The studied rocks range from Pliocene to Pleistocene clays and marls of southern Italy to Palaeozoic mudstones and greywackes of the Rhenohercynian Zone of the Czech Republic.The magnetic anisotropy of studied rocks is predominantly carried by the paramagnetic phyllosilicates, i.e. chlorite and, to the minor extent, micas. The orientation tensors of the phyllosilicate (001) planes were calculated from the neutron goniometry pole figures. Subsequently, the principal values of the theoretical anisotropy were calculated from the principal values of the orientation tensor assuming the various anisotropy values for the phyllosilicates grains. As only the paramagnetic anisotropy should be correlated with the preferred orientation of phyllosilicate phases, the HFA was used for the separation of the paramagnetic and ferromagnetic contribution to the magnetic anisotropy.In most cases, the principal directions of the AMS, high-field paramagnetic component (HFP), and the theoretical anisotropy are subparallel (Fig. 1). No systematic deviation of paramagnetic fabric from whole-rock magnetic fabric can be observed.Quantitative correlations were presented in terms of the standard deviatoric susceptibility, k’, and the difference shape factor, U, expressing anisotropy degree and shape, respectively (Fig. 2). The degrees of the theoretical anisotropy, AMS, and HFP correlate very well (correlation coefficient, R > 0.95) implying nearly the same degree of anisotropy for all the employed methods. The correlations of the shapes of respective anisotropies show more significant scatters. Despite this dispersion, the prolate and oblate shapes still remain well defined.This integrated approach enables to establish a more accurate qualitative and quantitative correlation between the phyllosilicate fabric and magnetic anisotropy and yields valuable information about the meaning of the magnetic fabric