Trapped in the rigid structure of diamonds formed deep in the Earth's crust, scientists have discovered a form of water ice that was not previously known to occur naturally on our planet.
The finding, published Thursday in Science, represents the first detection of naturally occurring ice-VII ever found on Earth. And as sometimes happens in the scientific process, it was discovered entirely by accident.
In normal ice, known as ice-I, the oxygen atoms arrange themselves in a hexagonal shape. In ice-VII these atoms are arranged in a cubic shape.
“Actually, there are several known phases of water ice that form under different pressure and temperature conditions.” explained Oliver Tschauner, a professor of geoscience at the University of Nevada, Las Vegas.
Water ice has very low comprehensibility. When it gets subjected to too much pressure, the atoms don't scooch together. Instead, they rearrange themselves into different patterns.
For example, if you press down hard enough on ice-I, it will transform into ice-II, which has a rhombohedral structure. Increase the pressure once again and the atoms will rearrange themselves into ice-III, then IV, V, VI and VII.
Unlike the other phases of ice, however, ice-VII remains fairly stable even as the pressure increases.
Scientists believe that ice-VII may be found in great abundance in the solar system, perhaps in the interior of ice moons like Enceladus and Europa, or as part of the ocean floor of Titan. But they did not think it could naturally occur on Earth.
Previous work has shown that ice-VII can be synthesized in the lab, but the new study revealed that small amounts of the material can also form naturally here on Earth, thanks to the peculiar properties of diamonds.
Diamonds can form very deep in the Earth's mantle, as much as 400 miles beneath the crust. As part of their formation process they will occasionally encapsulate teeny bits of the chemical environment around them in what are called inclusions.
What's special about inclusions in diamonds is that the material entrapped within them remains under the same pressure as it was during the time it was encapsulated.
However, as the authors discovered, diamonds can trap small bubbles of extremely dense pressurized water when they form. Then, as the diamond moves up through the mantle, the water inclusion is subjected to cooler temperatures while remaining under the same pressurized conditions. In that very specific case, ice-VII can occur.
Tschauner candidly admits that he and his team did not intentionally set out to look for ice-VII in diamonds. Instead, they were hunting for an unusual phase of carbon dioxide.
But while they were scanning the diamonds with high intensity X-rays, they saw something else: The first conclusive evidence of ice-VII on the planet.
"We were all very excited about that," Tschauner said.
Thanks to their discovery, ice-VII has been recognized for the first time as a mineral by the International Mineralogical Assn.
By DEBORAH NETBURN