Single‐domain particles in rocks and magnetic fabric analysis

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Abstract
Magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks, and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes. We have recently demonstrated however in laboratory produced samples that, as theoretically expected, susceptibility anisotropy is dependent on particle size. Multidomain particles of magnetite, or similar strongly magnetic particles with shape anisotropy, exhibit a maximum susceptibility parallel to their easy (remanent) magnetization axes whereas uniaxial single‐domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes. Hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state. By demonstrating uniaxial single‐domain anisotropy characteristics for the first time in a rock specimen, we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro‐ or ferrimagnetic particles in rocks. In particular, apparently foliated rocks can actually be lineated (and vice‐versa). Moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence. Remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment.