Ringwoodite

Ringwoodite is a mineral. It is a type of olivine which is formed at high temperatures and pressures in the Earth's mantle between 525 and 660 km deep. Olivine is a magnesium iron silicate. This mineral was first identified in a meteorite in 1969. It may be present in large quantity in the Earth’s mantle.
Ringwoodite is notable because to contains hydroxide ions (oxygen and hydrogen atoms bound together) in its structure. This suggests that there is from one to three times the world ocean's equivalent of water in the mantle's transition zone from 410 to 660 km deep.