User:Geo jt/sandboxes/Silicate perovskite

Stability range
Bridgmanite is a high-pressure polymorph of enstatite, but in the Earth predominantly forms, along with ferropericlase, from the decomposition of ringwoodite (a high-pressure form of olivine) at approximately 660 km depth, or a pressure of ~24 GPa. The depth of this transition depends on the mantle temperature; it occurs slightly deeper in colder regions of the mantle and shallower in warmer regions. The transition from ringwoodite to bridgmanite and ferropericlase marks the bottom of the mantle transition zone and the top of the lower mantle. Bridgmanite becomes unstable at a depth of approximately 2700 km, transforming isochemically to post-perovskite.

Calcium silicate perovskite is stable at slightly shallower depths than bridgmanite, becoming stable at approximately 500 km, and remains stable throughout the lower mantle.

The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO3 and CaSiO3 had been synthesized experimentally before 1975. By the late 1970s, it had been proposed that the discontinuity at about 660 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.

In 2004 it was proposed that silicate perovskites experience a further change in structure below about 2700 km to post-perovskite. This change is thought to explain the presence of the D" layer in the lowermost mantle.

Chemistry
The partitioning of Fe between magnesium perovskite and ferropericlase under lower mantle conditions has been extensively studied experimentally. The effects of varying the amount of Al in the silicate perovskite structure have also been studied.

===Abundance === Bridgmanite is the most abundant mineral in the mantle. The proportions of bridgmanite and calcium perovskite depends on the overall lithology and bulk composition. In pyrolitic and harzburgitic lithogies, bridgmanite constitutes around 80% of the mineral assemblage, and calcium perovskite < 10%. In an eclogitic lithology, bridgmanite and calcium perovskite comprise ~30% each.

Silicate perovskite is thought to be the main constituent of the lower mantle, possibly reaching up to 93% by volume. Magnesium silicate perovskite is probably the most abundant mineral phase in the Earth. The highest proposed abundances of silicate perovskites suggest that the lower mantle is richer in silica than the upper mantle and are consistent with the overall chondritic composition of the Earth.