Electronic spin transition of iron in the Earth's deep mantle

spin is a quantum property of every electron, associated with its intrinsic angular momentum. Though there are no suitable physical analogies to describe the spin quantum number, there are two possibilities , called spin 'up' and spin 'down.' The electronic structure of iron in minerals is generally such that valence electrons will more abundantly occupy different spatial orbitals and maintain the same spin than occupy the same spatial orbital and assume opposite spin, called 'spin-paired.' To the astonishment of mineral physicists , pressure-induced electronic spin-pair-ing transitions of iron that were predicted nearly 50 years ago recently have been detected in major mantle-forming oxides and silicates in ultrahigh-pressure experiments at lower-mantle pressures 2005]. If such a spin transition is occurring in the Earth's lower mantle, there may be profound geophysical implications. This article describes what is known about the nature of the spin transition, focusing on the possible effects on the physical properties of the deep mantle such as seismic velocities and transport properties , as well as on the effects of pressure and temperature on the spin transition interval (Figures 1 and 2). Recent experimental observations and theoretical advances are motivating multidisciplinary efforts to reevaluate the implications of spin transitions on our understanding of the state of Earth's lower mantle. Mineralogical models of the planet indicate that the lower mantle, the most voluminous layer of the Earth, consists of approximately 20% ferropericlase [(Mg,Fe)O] and approximately 80% silicate perovskite [(Mg, Fe)SiO 3 ] containing minor amounts of aluminum , in addition to a small amount of calcium silicate perovskite (CaSiO 3). Recent studies also show that silicate perovskite may transform to a post-perovskite structure just above the core-mantle boundary (see Eos 86(1) pp. 1,5, 2005). Iron (Fe) is the most abundant 3d transition metal in the mantle, substituting for magnesium (Mg) at a level of about 20% in ferropericlase and about 10% in silicate perovskite. The unique properties of iron give rise to complex physical and chemical properties of Earth's lower mantle. Iron exhibits two main valence states in silicates and oxides: ferrous iron (Fe 2+) with six 3d electrons and ferric iron (Fe 3+) with five 3d electrons. The electronic configuration of iron therefore depends on its oxidation state, that is, the number of valence electrons. The spin transition in iron results primarily from the competition between two quantities: the crystal field splitting energy (Δ) and the exchange splitting energy …