The spin state of Fe(II) and Fe(III) at temperatures and pressures typical for the Earth's lower mantle is discussed. 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.