At shallow depths, minerals with similar crystal structures often merge to become individual minerals, usually in a high-temperature environment. Despite the structural similarity, however, existing studies have shown that calcium-rich davemaoite and magnesium-rich bridgmaonite remain distinct throughout the lower mantle. “Why don’t davemaoite and bridgmanite merge into one, despite having very similar atomic-scale structures?” Sang-Heon Dan Shim, co-author of the Nature paper, said in a media statement. “Many attempts have been made to find conditions where these two minerals merge, but the answer from experiments has consistently been two separate minerals. Here we felt we needed some fresh new ideas in experiments.” The new experiment was an opportunity for the research team to test different heating techniques to compare methods. Instead of raising the temperature slowly in conventional high-pressure experiments, they raised the temperature very quickly to the high temperature associated with the lower mantle, reaching 3000–3500 F within a second. The reason for this was that once two minerals with a perovskite structure are formed, it becomes very difficult for them to fuse together even if they enter temperature conditions where a single perovskite mineral should be stable. By heating the samples rapidly to the target temperatures, Shim and co-author Byeongkwan Ko were able to avoid the formation of two minerals with a perovskite structure at low temperatures. Once they reached the temperature of the lower mantle, they monitored the minerals that formed for 15-30 minutes using X-rays. They found that only one perovskite mineral formed, unexpected from previous experiments. They also observed that at sufficiently high temperatures greater than 3500 F, davemaoite and bridgmanite become a single mineral in the perovskite-like structure. “It was thought that a large size difference between calcium and magnesium, the main cations of davemaoite and bridgmanite, respectively, should prevent the fusion of these two minerals,” Ko said. “But our study shows that they can overcome such a difference in warm environments.” The experiments suggest that the deeper lower mantle with a sufficiently high temperature should have a mineralogy different from the shallower lower mantle. Because the mantle was much warmer on the early Earth, the team’s new results show that most of the lower mantle then had a mineral with a perovskite structure, meaning the mineralogy was different from today’s lower mantle. This new observation has a number of implications for our understanding of the deep earth. Many seismic observations have shown that the properties of the deeper lower mantle differ from the shallower lower mantle. The changes are said to be gradual. The fusion of bridgmanite and davemaoite appears to be gradual in the research team’s experiments. Also, the properties of a rock with three main minerals, bridgmanite, ferropericlase, and davemaoite, do not match well with the properties of the deeper lower mantle. Ko and his colleagues predict that these unresolved problems can be explained by the fusion of bridgmanite and davemaoite into a new mineral with a perovskite structure.
