Ice-VII

ice-VII

diamond

Earth's mantle

davemaoite

Images

Formula: H2O
Oxide, paramorph of ice
Specific gravity: About 1.65
Environments: As inclusions in diamonds sourced in the Earth's mantle

Different types of ice have been created in the laboratory at different temperatures and pressures, and designated ice-I to ice-XVIII. Ordinary ice (ice-I) that freezes in puddles in cold climates used to be the only one recognised as a mineral by the IMA. A few years ago, however, ice-VII was found in nature and approved as a mineral in 2017.
Ice is hexagonal with specific gravity 0.9167; ice-VII is isometric with specific gravity about 1.65.
Ice-VII is unique in that it remains stable even as pressure increases dramatically. There’s almost nowhere on Earth for ice-VII to form, because it requires both low temperatures and high pressure exceeding 30,000 atmospheres (3 gigapascals). The only place you can reach that pressure is deep in the Earth’s mantle; it’s too hot for ice to form there, but the conditions are right for diamonds. Inclusions in diamonds are not uncommon, sometimes affecting the quality or colour of the diamond, but sometimes the inclusion is just water. The internal structure of diamonds does not change when they leave the high-pressure mantle, so, the water inside a diamond remains compressed, even when the diamond reaches the surface. The formation of ice-VII doesn’t require freezing temperatures — as long as the pressure is high enough, ice-VII can form at room temperature (https://www.extremetech.com/extreme/265377-scientists-find-ultra-rare-ice-vii-earth-first-time-inside-diamonds).

Localities

At the type locality, the Orapa Mine, Orapa, Letlhakane, Central District, Botswana, ice-VII was identified as an inclusion in a diamond. Indications are that it was formed in the transition zone of the mantle, from between 410 and 660 km depth (Science 359.1136).
Davemaoite was similarly found as an inclusion in a diamond from this locality.

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