MINERALS AT HIGH-PRESSURE

The crust and upper part of the mantle of the Earth are dominated by p hases with silicon in tetrahedral coordination with oxygen, whereas, i n the lower part of the mantle, silicon is in octahedral, or higher, c oordination. The goal of high-pressure mineralogy is to study the tran sitions 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 loca ted. Particularly large changes occur at the crust-mantle and mantle-c ore boundaries; however, additional density contrasts are found at 400 and 670 km, which define the transition zone between the upper and lo wer mantle. From high-temperature, high-pressure experiments and estim ates of the pressure-temperature gradient in the planet, the upper bou ndary of this zone corresponds to the breakdown of olivine, and the lo wer boundary is formed by the spinel to silicate perovskite transforma tion. Many of the effects associated with pressure increase, such as b ond compression and transformations to higher coordination numbers, ar e not surprising; however, other effects that seem to be associated wi th ordering are unexpected. Among these are the crystallization of ver y complicated structures with simple chemistry. Apparent violations of some of the crystal-chemical rules formulated at low pressure are als o observed in some phases.