for rock magnetism, they have remanence, so they can record the Earth's field. Iron does not occur widely in its pure form. It is usually incorporated into iron oxides, oxyhydroxides and sulfides. In these compounds, the iron atoms are not close enough for direct exchange, so they are coupled by indirect exchange or superexchange. The result is that the crystal lattice is divided into two or more sublattices with different moments. Ferrimagnetism Ferrimagnets have two sublattices with opposing moments. One sublattice has a larger moment, so there is a net unbalance. Magnetite, the most important of the magnetic minerals, is a ferrimagnet. Ferrimagnets often behave like ferromagnets, but the temperature dependence of their spontaneous magnetization can be quite different. Louis Néel identified four types of temperature dependence, one of which involves a reversal of the magnetization. This phenomenon played a role in controversies over marine magnetic anomalies. Antiferromagnetism Antiferromagnets, like ferrimagnets, have two sublattices with opposing moments, but now the moments are equal in magnitude. If the moments are exactly opposed, the magnet has no remanence. However, the moments can be tilted (spin canting), resulting in a moment nearly at right angles to the moments of the sublattices. Hematite has this kind of magnetism. Magnetic mineralogy Types of remanence Magnetic remanence is often identified with a particular kind of remanence that is obtained after exposing a magnet to a field at room temperature. However, the Earth's field is not large, and this kind of remanence would be weak and easily overwritten by later fields. A central part of rock magnetism is the study of magnetic remanence, both as natural remanent magnetization (NRM) in rocks obtained from the field and remanence induced in the laboratory. Below are listed the important natural remanences and some artificially induced kinds. Thermoremanent magnetization (TRM) When an igneous rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see isothermal remanence). This remanence can also be very stable, lasting without significant change for millions of years. TRM is the main reason that paleomagnetists are able to deduce the direction and magnitude of the ancient Earth's field. If a rock is later re-heated (as a result of burial, for example), part or all of the TRM can be replaced by a new remanence. If it is only