Scientists detected the first ever samples of naturally occurring ice-VII on Earth inside Diamonds.
The common ice is called ice-I, which has hexagonal crystal arrangement that causes it to have lower density than water. Compressing ice can change the shape of the crystals, turning ice-I into ice-II (rhombus-shaped crystals), ice-III (tetragonal crystals), and so on.
Ice-VII has cubic crystal arrangement with 1.5 times denser than ice-I.
It requires both low temperature and high pressure exceeding 30,000 atmospheres (3 gigapascals) for ice-Vll to form. The only place such pressure can be achieved is deep in the Earth’s mantle, but the temperature is very high for ice to form there.
Diamonds often pick up molecules during their formation deep in the Earth. The trapped water inside can become super-rare ice-VII in such high pressure.
