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 the ir distribution anisotropy through magnetic interactions between neigh bouring grains. Measurement of the low-field magnetic anisotropy of si ngle 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 typ es of grain arrangement: an ''aligned'' configuration and a ''side-by- side'' configuration. For a distance between the grain centers equal t o approximately twice the average grain size, the magnetic susceptibil ity and its anisotropy are enhanced in both configurations, and the di rection of k(max), the easiest magnetization axis, is stable in the '' aligned'' configuration, whereas it rotates toward an orthogonal direc tion in the ''side-by-side'' configuration. Depending on the distribut ion of the interacting magnetite grains, magnetic interactions may the refore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.