“This paper reports an unexpected change in the oxidation state of Fe in bridgmanite, the most dominant mineral in the lower mantle. The oxidation state change resolves the discrepancy between laboratory and seismic studies on the chemical composition of the lower mantle, showing that the lower mantle has major element chemistry similar to the upper mantle. The oxidation state change will also lead to a lower Fe content in bridgmanite in the midmantle, whereas the total Fe content remains the same. Such a change can lead to an increase in viscosity at 1,100- to 1,700-km depths, providing a viable mineralogical explanation on possible viscosity elevation suggested by geophysical studies at the same depth range.”
Check out this cool paper in this link.