A laboratory technique for studying the behavior of minerals under high-pressure conditions.
The nature of minerals as they exist at atmospheric pressure represents only a very limited part of their real nature. The range of pressure and temperature prevailing at the surface of the Earth is very limited compared to the ranges that exist in the other planets of the solar system. The bottom of the ocean, which is at the highest pressure that can be observed directly, is only 0.1GPa (1kilobar), while the pressure at the center of the Earth is 390GPa (3900 kilobars). Pressures at the centers of large planets such as Saturn and Jupiter exceed 1000 GPa (10,000 kilobars). Therefore, to study the formation and structure of the Earth and other planets, it is essential to study the behavior of minerals under high pressure. It has become clear through high-pressure experiments that the minerals constituting the Earth's lower mantle (which extends from 650 to 2900 km or 400 to 1800 mi from the surface and occupies more than 507percnt; of the entire volume of the Earth) are mostly so-called silicate perovskites that can never be formed on the surface of the Earth. See also Earth interior; Jupiter; Saturn.
Pressure is defined as a force per unit area; therefore, in order to apply a high pressure, it is necessary to concentrate a large force in a small area. Because of the limited strength of materials used to produce sample chambers, many different techniques are required, depending on the pressure range (see illustration).
Diagram of pressure (P) and temperature (T) within the Earth, showing capabilities of various types of high-pressure apparatus. 1 GPa = 10 kilobars. 1 km = 0.6 mi.
A large number of phase transformations has been found in minerals under high pressure, but most of these structures have already been observed in other minerals existing under atmospheric pressure. For example, rutile-type SiO2 (stishovite) is formed only above 10 GPa (100 kilobars), but the same structure is obtained at atmospheric pressure when the Si ion is replaced by the larger germanium (Ge) ion. This implies that crystal structure is determined mainly by the ratio of the cation radius to that of the anion.
When the very dense structure is compressed further, the bond length becomes shorter and shorter, and the orbitals of the electrons around the ions begin to overlap. This means that the orbital electrons can move freely in the material, which changes into a so-called metallic state. This metallic transition is believed to occur in all materials when they are subjected to high enough pressure. Even hydrogen, helium, and ice are believed to exist in the metallic state in the interiors of Jupiter and Saturn. In the laboratory, however, this transformation into the metallic state under pressure has been confirmed in only a limited number of materials such as Si, Ge, and gallium arsenide (GaAs). See also Bond angle and distance; Free-electron theory of metals.
It has become clear that many of the major phases of silicate transform into the perovskite structure above 25 GPa (25kilobars). Therefore it is believed that silicate perovskite is the most abundant mineral within the Earth, although it is an exotic mineral on the surface. In order to clarify the nature of the high-pressure minerals believed to be present in the interior of the Earth many studies have been made using various techniques. For this type of study, it is important to obtain a single crystal. The multianvil apparatus has been widely used for such experiments, and various single crystals of high-pressure minerals such as silicate perovskite, spinel, and stishovite have been synthesized.
High-pressure synthesis is a powerful method not only for use in earth and planetary sciences but also for the creation of new materials. Many industrial diamonds are synthesized by using high-pressure techniques, and some new high-pressure materials, such as cubic boron nitride, have found wide application. Pressure is one of the most fundamental parameters that can alter the state of materials, and research in this field is also expected to expand in the future. See also Diamond; High-pressure chemistry; High-pressure physics; Silicate phase equilibria; Solid-state chemistry; Solid-state physics.
