Gazing through the layers that make up the Earth—the crust, the upper mantle, the lower mantle, and the core—is no easy task. After all, it’s not as if a cross-section of the planet is readily available for study.
Scientists now believe they have discovered a new mineral in the lower mantle, a part that makes up 55 percent of the Earth’s total volume.
We already know about the three main minerals of the class – bridgemanite, ferroperclase and daphemaite – but there could be another mineral. What has puzzled researchers for some time is that bregmanite and daphemaite should theoretically fuse together at high temperatures, thanks to their similar crystal arrangements (technically known as the perovskite structure).
However, previous experiences have not shown this to be the case.
“Why don’t davemaoite and bridgmanite merge into one despite the fact that they have very similar atomic structures? This question has fascinated researchers for two decades,” says geologist Dan (Sang Hyun) Shim of Arizona State University.
“Many attempts have been made to find conditions in which these two minerals are combined, yet the answer from the experiments has always been two separate minerals. And that’s where we felt we needed some fresh new ideas in the experiments.”
Through a series of high-pressure heating experiments in a special chamber, the researchers attempted to simulate conditions throughout the lower mantle. Crucially, they ramped up temperatures very quickly, reaching around 1,650 to 1,925 degrees Celsius (3,000 to 3,500 degrees Fahrenheit) in less than a second, before rising to temperatures around 2,800 degrees Celsius.
The small, heated samples – now at temperatures representing the uppermost layer of the lower mantle all the way down to the deep lower mantle – have been observed by X-ray imaging to map the structure of their minerals. Unexpectedly, at temperatures approaching 2000 ° C and above, a single perovskite mineral appeared, which is a mixture of both davemaoite and bridgmanite.
This refers to the deeper part of the lower mantle that has a different mineral mixture than the upper part, due to higher temperature and pressure. Incorporating particulate and daphemaite at the bottom would add an additional mineral to consider, and the researchers think iron probably plays an important role.
“It is thought that the large size difference between calcium and magnesium, the major cations of daphemaite and bridgemanite, respectively, must have hindered the fusion of these two minerals,” says mineralogist Byeongkwan Ko of Michigan State University.
“But our study shows that they can overcome this difference in hot environments.”
Earth’s mantle was once much hotter than it is today, suggesting that fusion of perovskite minerals could have been more widespread—and thus the geologic makeup has changed dramatically over time.
These findings are consistent with previous research indicating that the properties of the lower mantle change as you go deeper. This is something that future studies will be able to analyze further – in terms of its current state and its state in the past.
“Our discovery requires a review of deep mantle mineralogy models and will have an impact on our understanding of the region’s formation, structure, dynamics and evolution,” the researchers wrote in their published paper.
The search was published in temper nature.
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