Silicate perovskite
Silicate perovskite is either or when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and mainly exist in the [lower Mantle (geology)|mantle (Earth)|lower part] of Earth's mantle, between about depth. They are thought to form the main mineral phases of the lower mantle, together with ferropericlase.
Discovery
The existence of silicate perovskite in the mantle was first suggested in 1962, and both and had been synthesized experimentally before 1975. By the late 1970s, it had been proposed that the seismic discontinuity at about 660 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.Natural silicate perovskite was discovered in the heavily shocked Tenham meteorite. In 2014, the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association approved the name bridgmanite for perovskite-structured, in honor of physicist Percy Bridgman, who was awarded the Nobel Prize in Physics in 1946 for his high-pressure research.
In 2021 perovskite-structured was found as an inclusion in a natural diamond. The name davemaoite has been adopted for this mineral.
Structure
The perovskite structure occurs in substances with the general formula, where A is a metal that forms large cations, typically magnesium, ferrous iron, or calcium. B is another metal that forms smaller cations, typically silicon, although minor amounts of ferric iron and aluminum can occur. 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 are in the orthorhombic crystal system.Occurrence
Stability range
Bridgmanite is a high-pressure polymorph of enstatite, but in the Earth predominantly forms, along with ferropericlase, from the decomposition of ringwoodite at approximately 660 km depth, or a pressure of about 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.