TRIVALENT CATIONS IN MANTLE SILICATE PEROVSKITE

In her 1999 Science paper, Alex Navrotsky addressed the importance of trivalent cations in mantle silicate perovskite. Here we report the effects of Fe³⁺ and Al³⁺ on Mg-silicate perovskite at high pressure. Perovskites with 0.09Fe₂O₃•0.91MgSiO₃ and 0.05Fe₂O₃•0.05Al₂O₃•0.9MgSiO₃ were synthesized above 50 GPa in the laser-heated diamond-cell. The spin state of Fe³⁺ was determined by synchrotron Mössbauer spectroscopy and X-ray emission spectroscopy at high-pressure. The effect of trivalent cation substitution on the unit cell volume of perovskite was investigated using synchrotron X-ray diffraction.

Above 50 GPa, Fe³⁺ and Al³⁺ enter the perovskite structure primarily by charge-coupled substitution. When Fe³⁺ exists alone in perovskite, it enters both the dodecahedral and octahedral sites. With compression, Fe³⁺ in the octahedral site undergoes a gradual change to the low spin state, while Fe³⁺ in the dodecahedral site remains high spin to at least 136 GPa. The spin transition results in a low bulk modulus for perovskite below 60 GPa.

When Fe3+ exists together with Al, at low pressure the majority of Fe³⁺ sits in the dodecahedral site in the high spin state. However, we found a rapid increase in the low spin Fe³⁺ fraction in the octahedral site of perovskite at ~70 GPa. Because low spin Fe³⁺ is similar in size to Al³⁺, more Fe³⁺ enters the octahedral site at higher pressure, while Al³⁺ must enter into the dodecahedral site. Above 80 GPa, approximately half of the Fe³⁺ exists in the octahedral site. As a result of the site mixing, the Al-bearing perovskite has a significantly smaller volume at lowermost mantle pressures.