Deep into the lower mantle: its new reactions, compositional changes, and structural layering under high temperature and high pressure

<p indent="0mm"> In this paper, we have summarized the progresses in mineralogical researches over the past two decades regarding to the structural phase transitions and mineral reactions in the lower mantle, and emphatically discussed the formation of Ca-rich bridgmanite and the influence of its structural phase transitions of (Al, H)-SiO₂, on the physical properties and chemical compositions of the lower mantle. Firstly, we have discussed the formation conditions of Ca-rich bridgmanite, clarified contradictions and problems in previous studies regarding to the influences of temperature, pressure, and elemental composition on the depth of the formation reaction of Ca-rich bridgmanite, suggested that this reaction could lead to stratification of phase compositions in the lower mantle, pointed out that its effects on the velocity, density, and element partitioning of the lower mantle should be further researched. By summarizing the influenece of Al and water on structural phase transitions of SiO₂ in subducted oceanic crust and the reactions between carbonates and SiO₂, we have further discussed relationships between these variations and the low velocity bodies in the middle part of the lower mantle at different depths and scales, and have proposed that further investigation of the influence of Al and water on the structural phase transitions of SiO₂ under relevant mantle temperature and pressure conditions is crucial for understanding the formation of small-scaled scatterers in the middle part of the lower mantle.