THE SPIN STATE OF IRON IN MINERALS OF EARTH'S LOWER MANTLE

The spin state of Fe(II) and Fe(III) at temperatures and pressures typical for Earth's lower mantle will be discussed. Using crystal field theory and an extended Bragg-Williams mean-field theory, we predict an extended high-spin to low-spin crossover region along the geotherm for Fe-dilute systems depending on crystal-field splitting, pairing energy, and cooperative interactions. In particular, spin transitions in ferromagnesium silicate perovskite and ferropericlase, the dominant lower mantle components, should occur in a wide temperature-pressure range. We also derive a gradual volume change associated with such transitions in the lower mantle. The gradual density changes and the wide spin crossover regions seem incompatible with lower mantle stratification resulting from a spin transition.