Among these three main minerals, two minerals bridgmanite and davemaoite both have perovskite type crystal structures. This structure is also widely known in physics, chemistry and materials engineering, as some materials with a perovskite-type structure have shown superconductivity.
At shallow depth, minerals with similar crystal structures often fuse and become single minerals, usually in a high-temperature environment.
Despite the structural similarity, however, existing studies have shown that calcium-rich davemaoite and magnesium-rich bridgmanite remain separate 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 release. “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 new ideas in experiments.”
The new experiment was an opportunity for the research group to test various 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 perovskite-structured minerals are formed, it is very difficult for them to fuse even if they enter temperature conditions where a single perovskite mineral should be stable.
By rapidly heating the samples to target temperatures, Shim and co-author Byeongkwan Ko were able to prevent the formation of two perovskite-structured minerals at low temperatures. Once they reached the temperature of the lower mantle, they monitored which minerals formed over 15-30 minutes using X-ray beams. They found that only one perovskite mineral formed, which was unexpected from previous experiments. They also noticed that at sufficiently high temperatures above 3500 F, davemaoite and bridgmanite become a single mineral in the perovskite-type structure.
“It has been 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 this difference in warm environments.”
Experiments suggest that the deeper lower mantle with a sufficiently high temperature should have a different mineralogy than the shallower lower mantle. Because the mantle was much warmer on early Earth, the group’s new results indicate that most of the lower mantle then had a single perovskite-structured mineral, meaning the mineralogy was different from today’s lower mantle.
This new observation has a number of impacts on our understanding of the deep earth. Many seismic observations have shown that the properties of the deeper lower mantle are different from those of the shallower lower mantle. The changes are reported to be gradual. The fusion of bridgmanite and davemaoite is shown to be gradual in the research group’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 collaborators predict that these unresolved issues can be explained by a fusion of bridgmanite and davemaoite into a unique new perovskite-structured mineral.