Shape anisotropy versus magnetic interactions of magnetite grains: Experiments and application to AMS in granitic rocks

The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low-field magnetic anisotropy of single multi-domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side-by-side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side-by-side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.