ringwoodite

ringwoodite, magnesium-iron silicate mineral with the chemical formula (Mg, Fe)₂SiO₄. It is a polymorph (a mineral with the same composition but a different crystal structure) of the rock-forming mineral olivine. Ringwoodite forms under high-pressure conditions and is therefore denser than other polymorphs of olivine. Although olivine occurs on Earth’s surface as the green gemstone peridot, ringwoodite is typically blue and occurs deep within Earth, at depths of about 410 to 660 km (250 to 410 miles) in the mantle transition zone, just above the lower mantle. At these depths it can store exceptionally large amounts of water within its crystal structure. It also forms under the extreme pressures generated by meteorite impacts. The mineral is named for the Australian geophysicist Alfred Edward Ringwood, who was known for his experimental research on high-pressure phase transitions in mantle minerals and for developing and testing models related to processes in Earth’s mantle.

Discovery and occurrence

Ringwoodite was first identified in 1969 in a fragment of the Tenham chondritic meteorite, which had fallen in Queensland, Australia, in 1879. Scientists predicted the existence of the mineral within Earth based on laboratory simulations of the intense pressures of the mantle (inspired by Ringwood’s experiments). However, direct confirmation did not come until 2014, when a microscopic inclusion of ringwoodite was discovered in a diamond from Brazil that had formed deep within the mantle. This inclusion was found to contain roughly 1–1.5 percent water by weight, not as liquid but in the form of hydroxide ions bound within the mineral’s structure. Separate research led by geophysicist Steven D. Jacobsen and seismologist Brandon Schmandt combined high-pressure laboratory experiments with studies of more than 2,000 seismograms (records of seismic waves generated by earthquakes) across the United States. Their results showed that the behavior of seismic waves was consistent with the presence of water and rock melting in the mantle transition zone, about 640 km (400 miles) deep inside Earth. Together with evidence of water retention in the diamond-hosted ringwoodite specimen, the findings supported the inference that ringwoodite is present in the mantle transition zone.

Physical properties

The physical properties of ringwoodite are listed below:

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• Chemical formula: (Mg, Fe)₂SiO₄
• Physical state: Crystalline solid
• Crystal structure: Cubic
• Color: Deep blue to purple
• Transparency: Transparent to translucent
• Density: About 3.9 g/cm³
• Hardness: 7–7.5 (Mohs scale)

Geological significance

Ringwoodite’s ability to retain water is crucial to understanding Earth’s water cycle. If the water concentration observed in the diamond-hosted sample is widespread throughout the mantle transition zone, the total amount of water stored there could be nearly three times the volume of water in Earth’s surface oceans.

Although the water cycle is commonly understood as a surface process involving evaporation, condensation, and precipitation, geologic evidence shows that water is also exchanged between Earth’s surface and interior through subduction and volcanic activity. The capacity of ringwoodite to store water supports the concept of a deep water cycle, in which oceanic water carried into the mantle at subduction zones can be retained at depth. Some of that water may later be released under changing pressure and temperature conditions in Earth’s interior and returned to the surface over geologic time.