The magnetization of natural materials

Abstract

It is very rare for the magnetization of complex materials, such as rocks, to have any one simple origin as the composition and grain sizes of their constituent ferromagnetic minerals are generally variable, even within a single specimen. Furthermore, the magnetic properties of such materials may well change with time as different minerals equilibrate to the conditions at the Earth’s surface, particularly if they begin to hydrate and oxidize (weather) in the Earth’s atmosphere. Unfortunately even the identification of the magnetic minerals present (Section 3.6) may be only of partial assistance in understanding the nature of the remanence as generally less than 5% (even<0.5%) of the total magnetic mineral content actually carries the remanent magnetization. This means that even rocks with abundant ferromagnetic minerals should still be physically regarded as a weak solution of ferromagnetic particles within a predominantly paramagnetic matrix. All remanent magnetizations (Table 4.1) are termed natural remanent magnetization (NRM) when they occur naturally, i.e. prior to any laboratory influences. In most materials, this natural remanence is divisible into at least two components, one of which was acquired at a specific time, for example at the time of formation of a rock or when a pot was fired. This component is termed a primary magnetization to distinguish it from any later acquired magnetizations, such as viscous remanences (Section 2.3.3), that are termed secondary magnetizations.