FIGURE 2.18 Influence of pressure on the iron atom. Shown is the predicted charge density of the doubly charged iron cation (Fe) in the mineral ferropericlase (Mg,Fe)O, in which it is surrounded by six oxygens (O). (Left) At low pressure the spins of the d electrons are maximally aligned, producing a net magnetic moment on each iron atom (called the high-spin or HS state) and the magnetic properties that we are familiar with, such as the tendency of magnetic minerals to align with the magnetic north pole. (Right) At high pressures characteristic of Earth’s deep mantle the spins pair (called the low-spin or LS state), the atomic magnetic moments vanish, and iron-bearing minerals are nonmagnetic. The figures show that the size and shape of the iron cation also change across the high-spin to low-spin transition: iron is smaller (by about 10 percent in volume) and less spherical in the low-spin state, which should produce a change in density and other physical properties of iron-bearing minerals. SOURCE: Tsuchiya et al. (2006). Reprinted with permission. Copyright 2006 by the American Physical Society.