A new form of water ice discovered – a quadrangular variant of the seventh ice can be found in the Earth’s mantle and on exoplanets

Strange high-pressure ice: Researchers discover a new type of water ice – Ice VIIt. This crystal form, made up of cubic basic units, develops from a cubic crystal structure at a pressure of about five gigapascals, as indicated by high pressure experiments. This type of water ice could thus occur in Earth’s upper mantle, but also in the crust and in the water-rich mantle of exoplanets, scientists report.

Water is a very special substance, because this molecule exhibits some properties in both liquid and crystalline form. These include in addition to Abnormality intensity or the ability to self disintegration There are also about 20 known forms of water ice. The crystal structure of these things varies greatly depending on pressure and temperature – the range runs from natural hexagonal ice crystals to square shapes and up to cage-like structures.

From cubic to tetrahedral crystal structure

Researchers led by Zachary Grande of the University of Nevada in Las Vegas have discovered another type of water ice. In their experiment, they subjected a sample of water ice to increasing pressure in a diamond seal piston. At regular intervals, the sample was temporarily melted using a jet charger in order to crystallize again under the prevailing pressure conditions.

It turns out that at a pressure of about five gigapascals, water ice changes from the already known cubic crystalline shape Ice-VII to a hitherto unknown crystalline structure. In this quadrangular variant of Ice-VIIt, the atoms of water ice temporarily arrange themselves into cubic base units. This intermediate structure of water ice has not been observed before, as reported by Grandi and colleagues.

Also new in Eis-X

High-pressure experiments also revealed that another known form of ice, Ice-X, forms at much lower pressures than previously thought. This particularly hard and pressure-stable ice forms at about 30 gigapascals – about 300,000 times atmospheric pressure, and this value is nearly three times lower than previously assumed, the researchers reported.

“This result demonstrates that the transformation to the iconic Ice-X is occurring under much lower pressures than previously assumed,” says Grande’s colleague Ashkan Salamat. “This is an important piece of the puzzle that was previously missing and represents one of the most accurate measurements of water under these conditions to date.”

Related to Earth’s inner and outer planets

These new findings are less relevant to everyday life and the ice we can directly observe. However, the various forms of water ice play an important role in the formation and processes in the Earth’s interior, among other things. Because these variables of water ice differ slightly in their physical properties, they can shape the strength, availability, and responses of water at different depths.

According to Grandi and his team, the newly discovered form of ice, Ice VIIt, can be found primarily in the Earth’s upper mantle. But this strange quadrangular ice can also occur on other planets or moons. There, VIIt ice can form some water in the crust and upper mantle. (Physical Review B, 2022; doi: 10.1103/PhysRevB.105.104109)

Source: University of Nevada, Las Vegas

March 23, 2022

– Nadia Podebrigar