Post-perovskite

Article By:

Lay, Thorne Earth Sciences Department, University of California, Santa Cruz, California.

Last reviewed:2006

DOI:https://doi.org/10.1036/1097-8542.YB061180

Rocks in the Earth's crust and mantle are composed of minerals: crystalline structures having elements in ordered lattices. For a common mineral, such as olivine [(Mg,Fe)₂SiO₄], there is a stable crystal form that exists for the pressure and temperature conditions present near the Earth's surface. The physical properties of a mineral like olivine are determined by its composition and crystal structure. At higher pressures and temperatures, below 410 km (255 mi) deep in the mantle, olivine is not stable in its near-surface form, and (Mg,Fe)₂SiO₄ occurs in different mineral polymorphs, with denser packing of the elements. An olivine-composition mineral form, called wadsleyite, exists below 410 km deep in the mantle. Below 520 km (320 mi) depth, there exists an even more densely packed mineral form of olivine called ringwoodite. If an olivine-bearing rock sinks in the mantle in a subducting lithospheric plate, the olivine minerals undergo phase transitions over narrow pressure (depth) ranges, transforming from one mineral form to the next with increasing pressure and temperature conditions. At 650 km (400 mi) depth, olivine composition in the ringwoodite structure undergoes a different type of phase transition, called a disassociative transition, which forms two distinct minerals—magnesium-silicate perovskite [(Mg,Fe)SiO₃] and ferropericlase [(Mg,Fe)O]. Rising mantle material undergoes the reverse sequence of phase transformations. At each phase transition, physical properties of the olivine-bearing rock, such as the density, bulk modulus (incompressibility), and shear modulus (rigidity), abruptly change. This causes corresponding rapid increases in elastic (seismic) wave velocities at depths of 410, 520, and 650 km.

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