Magnetic minerals have fascinated man since they were first used as compasses by the Chinese over 4000 years ago. Their scientific study has given rise to the interrelated disciplines of mineral magnetism, rock magnetism, and paleomagnetism, which have contributed to some of the most important scientific discoveries of the last century and continue to be at the forefront of scientific investigation at the beginning of this one. Rock magnetism is concerned with understanding the processes by which rocks become magnetized in nature and the factors which influence their ability to maintain a faithful record of the Earth’s magnetic field over geological time (the reader is referred to Dunlop and Ozdemir 1997 for the definitive guide to this subject). Mineral magnetism aims to understand the physical, chemical, and thermodynamic consequences of magnetic ordering in minerals and how their magnetic properties are influenced by structural and microstructural changes associated with phase transformations. This is the subject of this chapter. A wide range of different transformation processes occur in magnetic minerals, including convergent and non-convergent cation ordering, vacancy and charge ordering, oxidation and reduction, reconstructive or inversion transformations, and subsolvus exsolution. No review of this size could adequately cover all these topics, and so I will describe a small number of examples which illustrate the general principles involved, concentrating on the magnetic properties of oxide minerals. The reader is referred to Coey and Ghose (1987) for a review which covers the magnetic properties of silicate minerals. Magnetic ordering describes the transition from a disordered (paramagnetic) arrangement of magnetic moments above the Curie temperature, T c, to an ordered arrangement of aligned magnetic moments below T c. Several theories describing the magnetic and thermodynamic consequences of magnetic ordering are in common use today. This section provides a brief description of the different …