Spin-pairing transitions of iron were predicted to occur within the Earth’s interior nearly 50 years ago, but only in the past few years has direct experimental evidence for such transitions at lower mantle conditions been reported. A high-spin to low-spin transition of Fe2+ in (Mg,Fe)O is now well established by both experimental and computational data to occur near 50 GPa at room temperature for lower mantle compositions. However in the Earth’s most abundant phase, (Mg,Fe)(Si,Al)O3 perovskite, the picture is not so clear.

X-ray emission (XES) and nuclear forward scattering (NFS) data present conflicting results on the location, number and sharpness of the transition(s), and whether Fe2+ or Fe3+ or both are involved [1-4]. To reconcile these observations, we undertook the first high-pressure high-temperature study of iron-containing silicate perovskite using combined Mössbauer, NFS and X-ray diffraction techniques to determine the spin state of iron in the dominant lower mantle phase.

Fig. 19: NFS data of Mg0.88Fe0.12SiO3 perovskite at a) 7 GPa, 290 K; b) 110 GPa, 290 K; c) 62 GPa, ~ 1000 K. At low pressure only high-spin Fe2+ is present, characterised by low QS, while intermediate-spin Fe2+ is stabilised by high pressure (b,c) and high temperature (c), recognised by high QS. In c) QS is reduced due to the temperature effect, but is still significantly higher than it would be for high-spin Fe2+.

We collected 119 57Fe Mössbauer and 32 NFS data sets of Mg0.88Fe0.12SiO3 and Mg0.86Fe0.14Si0.98Al0.02O3 perovskite at beamline ID18. Experimental conditions included using a resistively-heated diamond-anvil cell at pressures up to 110 GPa and temperatures up to ~ 1000 K, combined with high-resolution X-ray diffraction of several of the same sample loadings. All data show the appearance of a new component above 30 GPa with high quadrupole splitting (QS) whose stability is enhanced with increasing pressure, and in situ high-temperature data show that this component is also stabilised by higher temperatures (Figure 19). The transition involved in the sudden change of QS is purely electronic, since high-resolution X-ray diffraction data show no change in crystal structure up to 110 GPa. The most likely possibility is a spin transition, and a decrease in spin number with pressure in silicate perovskite was already found using XES [1,2]. Using the relative abundance of components derived from our Mössbauer data to calculate the spin number as a function of pressure and temperature, we find that a high-spin to intermediate-spin transition of Fe2+ agrees well with all XES data (Figure 20a and 20b). Intermediate-spin Fe2+ is also consistent with the high quadrupole splitting and large centre shift of the new component, which was observed in previous NFS data [3,4]. From the observed trend of spin number with temperature, we predict that Fe2+ in (Mg,Fe)(Si,Al)O3 perovskite will be predominantly in the intermediate-spin state throughout most of the lower mantle (Figure 20c).

Fig. 20: Effect of pressure on the average spin number of a) Mg0.88Fe0.12SiO3 perovskite and b) Mg0.86Fe0.14Si0.98Al0.02O3 perovskite calculated from room temperature Mössbauer spectra (circles) assuming assignment of the new component to intermediate-spin Fe2+. Previous XES data [1,2] are shown as triangles and inverted triangles, respectively. c) Effect of pressure and temperature on the average spin number of Mg0.88Fe0.12SiO3 perovskite. Blue circles (spin number indicated within) indicate Mössbauer data, while green circles correspond to NFS data, and all spin numbers show a consistent decrease with increasing pressure and temperature. Lines of equal spin number have negative slope, hence along the lower mantle geotherm Fe2+ is predicted to be predominantly in the intermediate-spin state.


Spin transitions affect the electronic structure of Fe2+, which in turn influences lower mantle properties such as radiative and electrical conductivity, rheology and iron partitioning. Much of our knowledge of lower mantle behaviour is based on experiments on quenched samples, yet spin transitions are reversible with respect to both pressure and temperature. The stability of intermediate spin-state Fe2+ in lower mantle perovskite is not the first surprising revelation for the Earth’s most abundant phase, and based on the implications of spin transitions for lower mantle properties, is probably also not the last.



[1] J. Badro et al., Science 305, 383 (2004).
[2] J. Li et al., Proc. Nat. Acad. Sci. 101, 14027 (2004).
[3] J.M. Jackson et al., Am. Miner. 90, 199 (2005).
[4] J. Li et al., Phys. Chem. Miner. 33, 575 (2006).

Principal publication and authors

C. McCammon (a), I. Kantor (a), O. Narygina (a), J. Rouquette (a,b), U. Ponkratz (c), I. Sergueev (c), M. Mezouar (c), V. Prakapenka (d), L. Dubrovinsky (a), Nature, submitted.
(a) Bayerisches Geoinstitut, Universität Bayreuth (Germany)
(b) now at: Université de Montpellier II, LPMC, Montpellier (France)
(c) ESRF
(d) Center for Advanced Radiation Sources, University of Chicago (USA)