Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments

Abstract

The effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility (AMS) of igneous rocks, despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals. Current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks. On this work, a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non-interactive fractions in a rock. Results of the model show that a variable proportion of magnetically interactive grains, the relative values of the bulk susceptibility, degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock AMS. Experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported, and supports the model predictions. Both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains, the contribution to the AMS of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed.