MINERALS AT HIGH-PRESSURE

The crust and upper part of the mantle of the Earth are dominated by phases with silicon in tetrahedral coordination with oxygen, whereas, in the lower part of the mantle, silicon is in octahedral, or higher, coordination. The goal of high-pressure mineralogy is to study the transitions that accomplish this transformation, and the properties of the phases that result. Since any density discontinuity in the Earth will give rise to a seismic signal, several such boundaries have been located. Particularly large changes occur at the crust-mantle and mantle-core boundaries; however, additional density contrasts are found at 400 and 670 km, which define the transition zone between the upper and lower mantle. From high-temperature, high-pressure experiments and estimates of the pressure-temperature gradient in the planet, the upper boundary of this zone corresponds to the breakdown of olivine, and the lower boundary is formed by the spinel to silicate perovskite transformation. Many of the effects associated with pressure increase, such as bond compression and transformations to higher coordination numbers, are not surprising; however, other effects that seem to be associated with ordering are unexpected. Among these are the crystallization of very complicated structures with simple chemistry. Apparent violations of some of the crystal-chemical rules formulated at low pressure are also observed in some phases.