A tiny diamond contains tiny crystals of a new mineral. It makes up 5% of the lower mantle.
Crystals of davemaoite in a diamond Aaron Celestian, Natural History Museum of Los Angeles County
A Botswana-based ancient diamond contains a rare mineral that was discovered deep within Earth. The mineral, called davemaoite, provides a rare window into deep-Earth chemical processes.
The diamond was originally found in Orapa, Botswana. It measures approximately 4 millimetres in width and weighs in at 81 mg. It was sold by a dealer to a scientist at California Institute of Technology in 1987. The scientist was studying diamonds and neither the dealer nor scientist knew how unique it was.
Oliver Tschauner from the University of Nevada in Las Vegas analysed the diamond. It is currently at the Natural History Museum of Los Angeles County. To find out more about the deep inner workings of Earth, he is looking for "super-deep" diamonds.
The majority of diamonds are found 120-250 kilometres under the surface, but the super-deep varieties are found in the Earth's lower mantle which is 660 kilometres beneath the surface.
Tschauner and his associates discovered tiny crystals of another mineral in the Botswanan Diamond. These crystals were then extracted using a laser. Mass spectrometry was used to determine their origin.
These tiny crystals were found to be a form calcium silicate, which was thought to exist in the lower mantle. However, it had never been seen before. It has a unique cubic arrangement, known as a "perovskite" structure.
This particular perovskite's atomic composition, which is mainly composed of calcium, silicon, and oxygen, shows that it was formed only under extreme conditions in the lower mantle. The pressure there is more than 200,000 times higher than at Earth's surface. Calcium silicate, which is a white mineral known as wollastonite and has needle-like crystals, can be found under surface conditions.
Tschauner and his coworkers named the calcium silicate mineral davemaoite after Ho-Kwang Mao, deep-Earth scientist at Carnegie Institution for Science in Washington DC.
Normally, davemaoite would fracture if it were brought up to the Earth's surface due to the huge drop in pressure. It was kept inside a diamond to preserve it on the long journey to Orapa. This was probably between 100 million and 1.5 billion years.
Tschauner says that the diamond was broken open but the davemaoite was left intact for about one second. Then we observed it bulge under the microscope, and then it turned into glass.
Davemaoite makes up approximately 5% of Earth's lower crust. This is significant because it is believed that radioactive elements such as uranium and thorium can be found in the mineral. They heat Earth as they decompose. David Phillips, a University of Melbourne researcher in Australia, says that "without these radioactive elements the Earth would have cooled now."
Phillips says that the Botswanan davemaoite also contained some sodium, and potassium. This is surprising considering these elements are believed to be found mainly in Earth's crust, and not in the mantle. This shows that the mantle is being regenerated from surface material.
Tschauner and other researchers continue to search for super-deep diamonds in hopes of discovering more hidden minerals. It is a difficult task, as there aren't easy ways to distinguish between deep and shallow origins of diamonds. Also, it's not possible to predict where the deepest varieties will be found. Tschauner says, "The problem is that we don't know where to look."
Journal reference: Science, DOI: 10.1126/science.abm4742