Ringwoodite

Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between 525 and 660 km (326 and 410 mi) depth. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).Ringwoodite is polymorphous with forsterite, Mg2SiO4, and has a spinel structure. Spinel group minerals crystallize in the isometric system with an octahedral habit. Olivine is most abundant in the upper mantle, above about 410 km (250 mi); the olivine polymorphs wadsleyite and ringwoodite are thought to dominate the transition zone of the mantle, a zone present from about 410 to 660 km depth.In meteorites, ringwoodite occurs in the veinlets of quenched shock-melt cutting the matrix and replacing olivine probably produced during shock metamorphism.Ringwoodite crystallizes in the isometric crystal system with space group Fd3m. On an atomic scale, magnesium and silicon are in octahedral and tetrahedral coordination with oxygen, respectively. The Si-O and Mg-O bonds are both ionic and covalent. The cubic unit cell parameter is 8.063 Å for pure Mg2SiO4 and 8.234 Å for pure Fe2SiO4.Ringwoodite compositions range from pure Mg2SiO4 to Fe2SiO4 in synthesis experiments. Ringwoodite can incorporate up to 2.6 percent by weight H2O.The physical properties of ringwoodite are affected by pressure and temperature. The calculated density value of ringwoodite is 3.90 g/cm3 for pure Mg2SiO4; 4.13 g/cm3 for (Mg0.91,Fe0.09)2SiO4 of pyrolitic mantle; and 4.85 g/cm3 for Fe2SiO4. It is an isotropic mineral with an index of refraction n = 1.768.