Ringwoodite is a high-pressure mineral made of magnesium, iron, silicon, and oxygen that forms deep inside the Earth, between 520 and 660 kilometers below the surface. It has the same chemical ingredients as olivine, the green mineral abundant in the upper mantle, but its atoms are rearranged into a denser crystal structure by the extreme pressures found at depth. What makes ringwoodite remarkable is its ability to trap water inside its crystal lattice, potentially making the deep Earth one of the planet’s largest water reservoirs.
Chemical Makeup and Crystal Structure
Ringwoodite’s chemical formula is (Mg,Fe)₂SiO₄. It belongs to the spinel group of minerals, meaning its atoms are packed into a tight cubic arrangement rather than the looser structure found in olivine at the surface. This reorganization happens because the crushing pressure hundreds of kilometers underground forces the same atoms into a more compact configuration.
Olivine, wadsleyite, and ringwoodite are all polymorphs of each other: identical in chemistry, different in structure. As pressure increases with depth, olivine first transforms into wadsleyite at around 410 kilometers, then wadsleyite converts to ringwoodite at about 520 kilometers. At approximately 660 kilometers, ringwoodite itself breaks apart into two separate minerals, bridgmanite and ferropericlase, marking the boundary between the transition zone and the lower mantle. That decomposition occurs at roughly 23.5 to 24.5 gigapascals of pressure and temperatures around 1,650°C.
Where Ringwoodite Exists
Ringwoodite is the dominant mineral in the lower half of Earth’s mantle transition zone, the region stretching from 410 to 660 kilometers deep. You’ll never find it naturally at the surface because it’s only stable under enormous pressure. Once brought to lower pressures, it reverts to other mineral forms. This makes studying it a serious challenge.
The mineral was first identified not from Earth’s interior but from a meteorite. In 1969, researchers R.A. Binns, R.J. Davis, and S.J.B. Reed described tiny purple grains, up to 100 microns across, in thin sections of the Tenham meteorite, which had fallen in Queensland, Australia. The violent shock of a collision in space had generated pressures high enough to transform ordinary olivine into the spinel structure. They named it ringwoodite in honor of Professor A.E. Ringwood of the Australian National University, who had predicted through laboratory experiments that such a high-pressure form should exist.
The First Terrestrial Sample
For decades after its discovery in meteorites, no one had found a piece of ringwoodite that actually came from inside the Earth. That changed in 2014, when a team led by geologist Graham Pearson reported a microscopic inclusion of ringwoodite trapped inside a diamond from Brazil. The diamond had formed deep enough to encapsulate a grain of the mineral before being carried to the surface by volcanic activity, preserving the ringwoodite like an insect in amber.
This was the first direct physical evidence that ringwoodite exists in Earth’s mantle. More recently, researchers analyzing a gem diamond from the Karowe mine in Botswana found ringwoodite alongside other lower-mantle minerals, pinpointing its origin to conditions matching the 660-kilometer discontinuity. These rare diamond inclusions remain the only way scientists can hold a piece of the deep transition zone in their hands.
A Deep Reservoir for Water
Ringwoodite’s most striking property is its capacity to store water. The mineral doesn’t contain liquid water the way an underground lake would. Instead, hydrogen atoms (originally from water molecules) slot into vacant spaces in the crystal lattice, bonding with oxygen to form hydroxide groups. In lab conditions, ringwoodite can hold up to about 3 weight percent water, meaning roughly 3% of its total mass is effectively locked-up water.
The scale of this matters enormously. Ringwoodite dominates a 140-kilometer-thick shell of rock wrapping the entire planet. If even 1% of that rock’s weight is water, it would amount to nearly three times the volume of all Earth’s surface oceans combined. The transition zone could be the planet’s single largest water reservoir, one that has remained stable over geological timescales and plays a central role in how water cycles between the surface and the deep interior.
Seismic data supports the idea that at least some of this capacity is filled. The speed at which earthquake waves travel through the transition zone is consistent with ringwoodite that contains meaningful amounts of water. Hydrated ringwoodite is slightly less dense and conducts seismic waves differently than its dry counterpart, and those differences show up in global seismic surveys.
Why It Shapes Our Understanding of Earth
Ringwoodite’s phase transitions define two of the most important boundaries inside the planet. The conversion of wadsleyite to ringwoodite produces the seismic discontinuity at 520 kilometers, and ringwoodite’s breakdown into bridgmanite and ferropericlase creates the sharp 660-kilometer discontinuity. These boundaries aren’t just academic markers. They influence how material moves through the mantle, because the density changes at each transition can either help or hinder convection currents that drive plate tectonics at the surface.
The water question has even broader implications. If the transition zone holds a significant fraction of Earth’s total water budget, it affects models of how oceans formed, how volcanoes supply water vapor to the atmosphere, and how the planet has maintained habitable conditions over billions of years. Water also lowers the melting point of rock, so a hydrated transition zone could help explain pockets of partial melt detected just below 660 kilometers.
Ringwoodite research extends beyond Earth, too. Scientists studying Mars use the elastic properties of ringwoodite and other high-pressure minerals to model what seismic waves should look like traveling through the Martian interior. The ratio of ringwoodite to other minerals at a given depth changes seismic wave speeds more than temperature or iron content does, making it a key variable in interpreting data from Mars landers.
Physical Appearance
In the rare samples available for study, ringwoodite appears as tiny purple or blue grains. The grains found in the Tenham meteorite were deep purple and isotropic, meaning they look the same regardless of the direction light passes through them, a hallmark of cubic crystal symmetry. Synthetic ringwoodite grown in the lab can range from colorless (pure magnesium end-member) to increasingly blue or purple as iron content rises. The crystals are extremely small, typically measured in microns, and have only been seen either in shocked meteorites or as microscopic inclusions in ultra-deep diamonds.