High-pressure minerals

Minerals are formed from a number of factors, including pressure. Some minerals are formed in the low to moderate pressures of the earth’s outer layers. Other minerals are produced under great pressure. One of the best-known minerals formed in such conditions is the diamond, which is transformed from deep in the earth’s mantle and brought to the surface through a number of geologic processes.

Basic Principles

The earth has myriad minerals, each of which is formed under particular circumstances. Some minerals are formed in the earth’s outer crust, combining the eroded elements and compounds produced when water dissolves rocks on the planet’s surface. Other minerals are formed in cooling magma as it moves outward toward the earth’s surface. Still others are formed in the mantle (the layer of hot, solid rock that rests between the earth’s core and outer crust), which is located about 28 meters (90 miles) below the planet’s surface.

In the last of these scenarios, minerals are produced under high amounts of pressure. This pressure comes from downward movement of the planet’s tectonic plates (massive slabs of rock that are in constant motion beneath the earth’s surface). The plates themselves are under pressure from gravity and the weight of oceans and glaciers, or are forced beneath another plate (a process known as subduction).

Minerals also may be produced under the intense pressure between tectonic plates. Here, other minerals and rock that have been pushed outward from the mantle may be caught between two contacting plates. In these cases, the pressure causes the rocks to change compositionally to form new minerals (a process known as metamorphism).

Still other types of minerals are formed not in the mantle but deep beneath the earth’s crust in sedimentary basins. Here, a number of environmental factors create different pressures that, in turn, cause varying types of mineral configurations.

Mineral Formation Under Pressure

A mineral is an amalgamation of basic elements, such as carbon, copper, iron, and oxygen. Some minerals comprise a single element. Minerals are solid in texture, formed in nature with crystalline structures. Each element contains atoms that bond with other atoms through the electronic charges that draw together the atoms’ respective electrons and protons. Thousands of minerals exist, developed under a variety of degrees of pressure and temperatures. Glass and ice are minerals, too, although ice ceases to be a mineral when it melts.

Two major natural forces—temperature and pressure—are significant in the formation of minerals. Temperature, for example, causes atoms to vibrate and to distance themselves from one another, causing instability in the crystalline structure and, as temperature increases, liquefaction. Pressure draws atoms together, creating more dense crystalline structures.

For a mineral to coalesce from the liquid and semisolid materials emanating from Earth’s core and mantle, high degrees of pressure (measured in atmospheres, the equivalent of approximately 14.7 pounds per square inch) are often essential. For example, muscovite (also known as mica) and feldspar—minerals that commonly form igneous rocks (rocks that form from cooling magma)—depend upon high levels of pressure for such bonding to occur.

Conditions in the Mantle

In many cases, high-pressure minerals are formed under the intense pressure and temperatures of the mantle. The mantle appears in two distinct parts: upper and lower. Conditions in each part (including pressure and temperature) are somewhat different. Therefore, different high-pressure minerals may be found in these two areas.

In the upper mantle (on which the earth’s tectonic plates rest), the temperature of the rock is at or near a melting point. Scientists believe this area produces the magma that is pushed out through volcanoes. Upper mantle minerals form under pressure from subducting plates above it. Scientists attempting to discern the types of minerals that are present in the upper mantle typically rely on examinations of material ejected from erupting volcanoes. One mineral, iron (which is found in the earth’s core), is typically disregarded in this pursuit. However, other minerals formed under high levels of pressure in the upper mantle include peridotite, eclogite, and diamonds.

The pressure in the lower mantle is far greater than found in the upper mantle. Scientists link the pressure in which a mineral is produced to its overall strength. Hence, minerals produced in the lower mantle tend to have greater strength than those in the upper mantle. Lower mantle minerals, such as perovskite, possess a much greater level of strength (including a resistance to higher temperatures) than upper mantle minerals such as peridotite.

Diamonds

One of the best-known high-pressure minerals is the diamond. One of the hardest materials on Earth, diamond is also among the most sought-after minerals. A common misperception is that diamonds are formed from coal. In reality, the process by which diamonds are formed provides an excellent illustration of the environment in which many high-pressure minerals are formed.

Diamonds are formed in a number of environments. The notion that diamonds are made from coal comes from the knowledge that diamonds, like a large percentage of minerals, consist of carbon. Diamonds that are formed in the earth’s lower mantle are derived from carbon that was deposited deep in the interior when the planet was still forming. The areas in which diamonds are formed are known as diamond stability zones, wherein the pressure and temperature is constant. Diamonds are dislodged from these zones during volcanic eruptions and are then carried into sedimentary basins from which they are mined.

The lower mantle is not the only place where diamond stability zones exist. Diamonds may be formed farther outward in the upper mantle. Here, however, the carbon from which they are formed comes from rocks such as limestone and marble (and organic debris, such as plants) that have been subducted by oceanic plates. Diamond stability zones are formed in these environments as oceanic plates are subducted under continental plates, creating extremely high pressure on the upper mantle (where the diamonds are formed). The churning effects of subduction move the diamonds, along with other rocks in the upper mantle, from stability zones and either out toward the surface or deeper into the mantle.

Metamorphism

A mineral’s assemblage and texture may be altered significantly because of pressure. In other words, higher levels of pressure may, in essence, significantly change the composition of the rock or mineral. This process is known as metamorphism. As pressure increases, so, too, does the rate of change in the mineral (through a concept called prograde metamorphism). Scientists believe there also exists a rare form of metamorphism in which, as pressure is decreased, a mineral’s assemblage may change. This latter manifestation is known as retrograde metamorphism.

Pressure that induces metamorphism is not of a singular nature; a number of different types of pressure can influence how a mineral is formed. For example, the shapes and crystalline structures of certain minerals are frequently altered because of varying degrees of pressure emanating from one side of the mineral or another. This differential stress can cause deformations such as rounding or disintegration into sheets. In other cases, the stress is uniform: It is applied in equal proportions from all around the mineral.

and Mineral Formation

The theory of plate tectonics (the notion that the earth’s lithosphere, located just below the outer crust, comprises a series of giant stone slabs that are in constant motion) reveals a great deal about how high-pressure minerals are formed and reformed. The concept demonstrates how molten material flows outward from the mantle toward the earth’s outer crust, contributing to the movement of the plates (which are also moving because of gravitational forces). Within the plate tectonics framework, thousands of the earth’s minerals (including high-pressure minerals) are created.

According to plate tectonics, basalt (an iron- and magnesium-rich form of rock that is low in density and is therefore commonly found in volcanic lava) flows out through the lithosphere and toward the plates. Contained in the basalt are the elements and minerals essential for the formation of other minerals. As the basalt arrives at the lithosphere, some of the minerals are crystallized, depending on the elements present and on environmental conditions (which include pressure).

In time, however, the minerals that have become conjoined with the rest of a tectonic plate eventually start to descend, largely because of the strong forces of gravity pushing down on the plates. As the plate descends, the minerals again undergo increasing levels of pressure, causing mineral deformation and metamorphosis.

An example of this process may be found in a study of the mountains of the Czech Republic. Geologists studying the geodynamic processes of this region found large concentrations of high-pressure mineral grains. These minerals provide evidence of the thickening of the plate suggested in plate tectonics. The study also revealed a diverse array of pressure environments. (Various concentrations of certain high-pressure minerals, such as eclogite and peridotite, found in the research illustrate that the region is in a dynamic state.) In other words, when minerals are formed under certain high-pressure conditions, they may be metamorphosed when the pressure equilibrium is upset as the plate is pushed down by gravity.

Subduction

A key process in the formation of minerals under high grades of pressure is subduction. In this phenomenon, tectonic plates are pushed down because of the presence of glaciers or oceans above them or because of pressure exerted upon them through contact with other plates. This process causes intense downward pressure on the mantle. It also causes extreme pressure in the areas (the subduction zones) between the plates.

Scientists can study the minerals formed in these high-pressure environments after the minerals are forced into subduction zones and out through volcanic activity. The configuration of the rocks containing these minerals often provides evidence of repeated metamorphism, demonstrating that high-pressure minerals are formed in a dynamic environment. For example, mineralogists and geologists studied a sample of eclogite found in an area of central China in which the South China block, a tectonic plate, is deeply subducted. The sample showed multiple decompression textures, revealing that the minerals had been formed, transferred, and reformed under the high pressure of the subduction zone.

High-Pressure Formation in Sedimentary Basins

As suggested earlier, minerals formed under great degrees of pressure are not necessarily located in the mantle. Indeed, a number of conditions exist in which minerals are formed and reformed closer to the outer crust. For example, studies of sedimentary basins (lower areas in the earth’s crust in which sedimentary rocks and minerals accumulate) show that high levels of pressure exist in certain areas, facilitating the formation of certain mineral deposits.

Geologists are increasingly focused on the presence of water and other liquids in a number of sedimentary basins. These liquid deposits, contained in pockets (or pores), exert pressure on the surrounding sediment. This liquid pressure may be in a state of equilibrium, a condition known as hydrostatic pressure. The fluid pressure is great enough to deform existing rocks and minerals and to form new ones.

If the pore is influenced by increases in temperature, increased compaction from additional sediment, or other factors (including volcanic activity), the fluids may experience a state of overpressure. Even human-caused compaction, such as exploratory mining for fuels and gems, can increase pressure on a pore. When the pore reaches an overpressure point, it may explode violently. The same fluid pressure that can form minerals in sedimentary basins also can cause large amounts of damage and fatalities.

Principal Terms

deformation: geologic process whereby the shape and size of rocks and minerals are altered because of the application of pressure, temperature, or chemical forces

equilibrium: geologic state in which temperature and pressure, along with other environmental conditions, are in balance

hydrostatic: state of fluid pressure equilibrium

mantle: layer of hot, solid rock between the earth’s core and outer crust

metamorphism: geologic process whereby igneous or sedimentary rocks are changed compositionally under high pressure

sedimentary basin: lower area in the earth’s crust in which sedimentary rocks and minerals accumulate

subduction: geologic process whereby a tectonic plate is forced below another plate

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