Silicate perovskites include the magnesium iron silicates (Mg,Fe)SiO3 (also known as bridgmanite) and the calcium silicate CaSiO3, which have a perovskite structure. Silicate perovskites are mainly found in the lower part of the Earth's mantle, below about 670 km (approximately 400 miles). They are thought to form the main mineral phases, together with ferropericlase.
In 2014, the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) approved the name bridgmanite for perovskite-structured (Mg,Fe)SiO3, in honor of the physicist, Percy Bridgman, who won the Nobel Prize in Physics, in 1946, for his high-pressure research.
The physical properties of silicate perovskites under lower mantle conditions, such as seismic velocity, are studied experimentally using laser-heated diamond anvil cells. Naturally occurring silicate perovskites cannot be studied as they are unstable at the Earth's surface.
The perovskite structure (first identified in the mineral perovskite) occurs in substances with the general formula ABX3, where A is a metal that forms large cations, B is another metal that forms smaller cations and X is typically oxygen. The structure may be cubic, but only if the relative sizes of the ions meet strict criteria. Typically, substances with the perovskite structure show lower symmetry, owing to the distortion of the crystal lattice and silicate perovskites in the orthorhombic crystal system.
Upper limit of stability
The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO3 and CaSiO3 had been synthesised experimentally before 1975. By the late 1970s, it had been proposed that the discontinuity at about 650 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.
Lower limit of stability
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.
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.
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.
Experimental deformation of polycrystalline MgSiO3 under the conditions of the uppermost part of the lower mantle suggests that silicate perovskite deforms by a dislocation creep mechanism. This may help explain the observed seismic anisotropy in the mantle.
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- Bridgemanite on Mindat.org
- Eos, Transactions American Geophysical Union, Volume 95, Issue 23, Article first published online: 10 June 2014 PDF
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