Low-temperature magnetic anisotropy in micas and chlorite

Phyllosilicates, such as micas and chlorite, are common rock-forming minerals and often show preferred orientation in deformed rocks. In combination with single-crystal anisotropy, this leads to anisotropy of physical properties in the rock, such as magnetic susceptibility. In order to effectively use the magnetic anisotropy to understand a rock fabric, it is necessary to identify the minerals responsible for the magnetic anisotropy. Techniques have been developed to separate contributions of the ferrimagnetic, antiferromagnetic, paramagnetic, and diamagnetic susceptibilities to the anisotropy of magnetic susceptibility. Because diamagnetic and paramagnetic susceptibility are both linearly dependent on field, separation of the anisotropic contributions requires understanding how the degree of anisotropy of the paramagnetic susceptibility changes as a function of temperature. Note that diamagnetic susceptibility is not dependent on temperature. The increase in paramagnetic anisotropy at low temperature is used to separate the paramagnetic and diamagnetic subfabrics, and can be expressed by the p(77) factor. In this study, we determined p(77), which is the change in the degree of anisotropy (delta k) between room temperature (298 K) and liquid nitrogen temperature (77 K), for a series of micas and chlorite. The paramagnetic susceptibility ellipsoid is highly oblate with the minimum principal susceptibility normal to the silicate layers at both 77K and RT. The degree of anisotropy delta k increases by a factor of approximately 6.3-8.7 for individual samples of muscovite, phlogopite and chlorite on cooling from RT to 77 K and between 11.2 and 12.4 for biotite. A decrease in temperature enhances the paramagnetic anisotropy in a mineral. Biotite exhibits a relatively stronger enhancement due to the onset of magnetic ordering below similar to 100 K. This can have important implications for interpreting low temperature anisotropy in mudstones, mica schists and gneisses. (C) 2014 Elsevier B.V. All rights reserved.