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

The effect of magnetic interactions has been commonly neglected in the inte rpretation of the anisotropy of magnetic susceptibility (AMS) of igneous ro cks, despite the likely occurrence of magnetic interactions due to their re latively high content of ferromagnetic minerals. Current models of magnetic interactions apply to highly simplified systems and are not directly appli cable to natural rocks. On this work, a model that includes variables previ ously neglected is used to assess the relative importance of magnetically i nteractive and non-interactive fractions in a rock. Results of the model sh ow that a variable proportion of magnetically interactive grains, the relat ive 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 i mportance of the distribution of the ferromagnetic grains within a specimen is also reported, and supports the model predictions. Both theory and expe riments indicate that if a rock has unequant magnetic gains and clusters of magnetic grains, the contribution to the AMS of the rock will depend on th e relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed. (C) 2001 Elsevier Science B.V All rights reserved.