Ringwoodite

Formula: SiMg₂O₄
Nesosilicate (insular SiO₄ groups), ulvöspinel subgroup, oxyspinel group, spinel supergroup, the isometric polymorph of orthorhombic wadsleyite, and forsterite, which is also orthorhombic
Crystal System: Isometric
Specific gravity: 3.9 calculated
Colour: Bluish grey to smoke-grey, purple, colourless. Synthetic material shines bright blue
Common impurities: Ti,Mn,Ca
Environments

Extraterrestrial environments
Deep Mantle environments

Ringwoodite occurs in veinlets cutting the matrix of meteorites and replacing ferroan grains of its polymorph forsterite (HOM, Webmin, Dana). It is probably produced during impact metamorphism. Associated minerals include majorite and magnesian silicate glass. (HOM). has also been found as inclusions in terrestrial diamonds (Mindat).

Localities

At the type locality, the Tenham meteorite, Tenham Station, Windorah, Barcoo Shire, Queensland, Australia, ringwoodite occurs in rounded grains up to 100 pm in diameter in black veins, and also as pseudomorph after olivine (AM 54.1219). Majorite is an associated mineral (Mindat).

Juína, Juína kimberlite field, Mato Grosso, Brazil. Diamonds form in the Earth's mantle and are brought to the Earth's surface in violent eruptions of kimberlites. At Juína a diamond from a 100-million-year-old kimberlite was found to contain ringwoodite, which is thought to form only between 410km and 660km beneath the Earth's surface. This region is the transition zone between the upper and lower mantle, and mainly comprises and wadsleyite. This is the first reported occurrence of a terrestrial ringwoodite, rather than a meteoric one, and, surprisingly, researchers found that the mineral contains about 2.5% of its weight of water, indicating that the transition zone is hydrous, to about 1% weight of water, and that implies that the interior of the Earth may store several times the amount of water in the oceans.
Ringwoodite is expected to form deep in other rocky planets, such as Mars.
The presence of hydrous ringwoodite in a diamond from transition zone depths supports the view that high fluid activity, notably that of water, has a key role in the genesis of ultradeep diamonds, and also provides a strong indication that some kimberlites must come from at least transition zone depths (Letter to Nature: Nature 507, 221–224 (13 March 2014), reported at https://www.bbc.com/news/science-environment-26553115).

At Xihe, Zengdu District, Suizhou, Hubei, China, ringwoodite is found in extraterrestrial microspherules of melt glass in granite (Dana).

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