Ringwoodite

High-pressure olivine polymorph with a spinel structure that is found in highly shocked meteorites (above ~50 GPa, shock level > S5) and the Earth’s transition zone mantle (~13 GPa). Under even higher pressure in the lower mantle (~24 GPa), ringwoodite decomposes into perovskite (Mg,Fe)SiO3, and magnesiowüstite (Mg,Fe)O, whose properties are completely different. This transformation explains the observed discontinuity between Earth’s upper and lower mantle. At lower pressure, ringwoodite transforms into wadsleyite, another olivine polymorph.

Ringwoodite in the Taiban meteorite. Image source: www.psrd.hawaii.edu

Natural ringwoodite is generally more Mg-rich,but, like olivine, it does form a continuous solid solution series whose end-members also range from pure Mg to pure Fe.  Iron‐enriched ringwoodite has been documented from only a handful of meteorites. Natural ringwoodite was first discovered in shock-melt veins within the Tenham meteorite in 1969: “The [ringwoodite] grains occur chiefly within black veins cutting across the stones, but the same material also replaces olivine within 10 – 20 microns of certain thicker veins and at the margins of some large chondritic fragments within the veins. Our investigations show that the purple mineral is the high pressure spinel polymorph of olivine, and for this first authentic natural occurrence we propose the name ringwoodite, in honour of the experimental studies by Professor A. E. Ringwood, Australian National University. The name covers the entire range of (Mg,Fe)2SiO4 spinels …”

Beyond its its initial discovery in chondrites, ringwoodite has also been found in and around shock veins within Martian and lunar meteorites. In an unusual twist of events, ringwoodite was found in a terrestrial specimen much later than it was found in meteorites. The first natural occurence of a terrestrial ringwoodite came in 2014, when a small ringwoodite crystal was found enclosed in a diamond whose primary source was in the deep mantle region, far lower than a typical diamond would normally originate.

Ringwoodite is able to contain hydroxide ions (OH) within its structure. As such, research suggests that vast quantities of water (H2O) may be trapped in the Earths transition zone mantle. Recent research has shown that both iron-free and iron-bearing ringwoodite can store about 0.8-1.2 wt.% of H2O under mantle transition zone conditions and temperatures of 1600 – 2000 K.

Ringwoodite can appear dark blue or purple in meteorites due to the absorbtion of red light that occurs during the exchange of an electron between the iron ions, Fe2+ and Fe3+.


Some or all content above used with permission from J. H. Wittke.