Ringwoodite
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Ringwoodite is a high-pressure phase of Mg₂SiO₄ formed at high temperatures and pressures of the Earth's mantle between 525 and 660 km depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite. Wikipedia
Crystal class: Hexoctahedral (m3m); H-M symbol: (4/m 3 2/m)
Crystal habit: Microcrystalline aggregates
Crystal system: Cubic
Formula (repeating unit): Magnesium silicate (Mg2SiO4)
IMA symbol: Rgd
Optical properties: Isotropic
Refractive index: n = 1.8