To most of us, ice is just ice. But scientists have categorized no less than 16 types, created under different conditions and featuring different crystalline structures. Most of these have only been created in the lab, but now, geoscientists at the University of Nevada Las Vegas (UNLV) have discovered a type known as Ice VII (seven) locked inside diamonds. This marks the first time the substance has been directly detected in nature, and it suggests that the Earth's mantle is home to huge pockets of water.
Ice's most common and familiar form is known, appropriately, as Ice I, but its phases are numbered right through to Ice XVI. Ice VII has a cubic crystalline structure, and can form at room temperature from water under 30,000 atmospheres of pressure. The stuff is well known in the lab and is believed to occur naturally on other planets and moons, such as Titan and Europa.
The new study marks the first time Ice VII has been found naturally on Earth, prompting the International Mineralogical Association to officially recognize it as a mineral. This unique crystallized water was found in the form of inclusions (impurities) in diamonds, which formed deep inside the Earth and were slowly pushed upwards over time.
The team says these diamonds would have formed in the mantle some 500 mi (805 km) below the surface, at temperatures over 1,000° F (538° C). The ice is solid now while confined inside the diamonds, but at those depths it would have existed as a liquid, which indicates that there is far more water in the Earth's mantle than previously believed. And since traces of Ice VII were found in diamonds from China, South Africa and Botswana, it appears to be a worldwide thing.
"These discoveries are important in understanding that water-rich regions in the Earth's interior can play a role in the global water budget and the movement of heat-generating radioactive elements," says Oliver Tschauner, first author of the study. "It's another piece of the puzzle in understanding how our planet works."
The researchers say the discovery was made by accident, as they hunted for carbon dioxide. The find should also help scientists build better models of how and where heat is generated deep inside the planet.
The research was published in the journal Science.
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