Mineral physics
Encyclopedia
Mineral physics is the science of materials that compose the interior of planets, particularly the Earth. It overlaps with petrophysics
, which focuses on whole-rock properties. It provides information that allows interpretation of surface measurements of seismic waves
, gravity anomalies, geomagnetic fields and electromagnetic
fields in terms of properties in the deep interior of the Earth. This information can be used to provide insights into plate tectonics
, mantle convection
, the geodynamo and related phenomena.
Laboratory work in mineral physics require high pressure measurements. The most common tool is a diamond anvil cell, which uses diamonds to put a small sample under pressure that can approach the conditions in the Earth's interior.
The diamond anvil cell
is a small table-top device for concentrating pressure. It can compress a small (sub-millimeter sized) piece of material to extreme pressure
s, which can exceed 3,000,000 atmospheres (300 gigapascals
). This is beyond the pressures at the center of the Earth. The concentration of pressure at the tip of the diamonds is possible because of their hardness, while their transparency
and high electrical conductivity allow a variety of probes can be used to examine the state of the sample. The sample can be heated to thousands of degrees.
varies with pressure
and temperature
. Such a relation is called an equation of state
(EOS). A simple example of an EOS that is predicted by the Debye model
for harmonic lattice vibrations is the Mie-Grünheisen equation of state:
where
is the heat capacity
and
is the Debye gamma. The latter is one of many Grünheisen parameters that play an important role in high-pressure physics. A more realistic EOS is the Birch–Murnaghan equation of state.
: plotting data for a large number of rocks, he found a linear relation of the compressional wave velocity
of rocks and minerals of a constant average atomic weight
with density 
:
.
This makes it possible to extrapolate known velocities for minerals at the surface to predict velocities deeper in the Earth.
Petrophysics
Petrophysics is the study of the physical and chemical properties that describe the occurrence and behavior of rocks, soils and fluids . Petrophysics mainly studies reservoirs of resources, including ore deposits and oil or natural gas reservoirs...
, which focuses on whole-rock properties. It provides information that allows interpretation of surface measurements of seismic waves
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
, gravity anomalies, geomagnetic fields and electromagnetic
Magnetotellurics
Magnetotellurics is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with...
fields in terms of properties in the deep interior of the Earth. This information can be used to provide insights into plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, mantle convection
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
, the geodynamo and related phenomena.
Laboratory work in mineral physics require high pressure measurements. The most common tool is a diamond anvil cell, which uses diamonds to put a small sample under pressure that can approach the conditions in the Earth's interior.
Creating high pressures
The pace of progress in mineral physics has been determined, to a large extent, by the technology for reproducing the high pressures and temperatures in the Earth's interior. The most common tools for achieving this have been:Shock compression
Many of the pioneering studies in mineral physics involved explosions or projectiles that subjected a sample to a shock. For a brief time interval, the sample is under pressure as the shock wave passes through. Pressures as high as any in the Earth have been achieved by this method. However, the method has some disadvantages. The pressure is very non-uniform and is not adiabatic, so the pressure wave heats the sample up in passing. The conditions of the experiment must be interpreted in terms of a set of pressure-density curves called the Hugoniot curves.Multi-anvil press
Multi-anvil presses involve an arrangement of anvils to concentrate pressure from a press onto a sample. Unlike shock compression, the pressure exerted is steady, and the sample can be heated using a furnace. Pressures equivalent to depths of 700 km and temperatures of 1500°C can be attained. The apparatus is very bulky and cannot achieve pressures like those in the diamond anvil cell (below), but they can handle much larger samples that can be examined after the experiment.Diamond anvil cell
Diamond anvil cell
A diamond anvil cell is a device used in scientific experiments. It allows compressing a small piece of material to extreme pressures, which can exceed 3,000,000 atmospheres ....
is a small table-top device for concentrating pressure. It can compress a small (sub-millimeter sized) piece of material to extreme pressure
Orders of magnitude (pressure)
This is a tabulated listing of the orders of magnitude in relation to pressure expressed in pascals....
s, which can exceed 3,000,000 atmospheres (300 gigapascals
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
). This is beyond the pressures at the center of the Earth. The concentration of pressure at the tip of the diamonds is possible because of their hardness, while their transparency
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
and high electrical conductivity allow a variety of probes can be used to examine the state of the sample. The sample can be heated to thousands of degrees.
Equations of state
To deduce the properties of minerals in the deep Earth, it is necessary to know how their densityDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
varies with pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. Such a relation is called an equation of state
Equation of state
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
(EOS). A simple example of an EOS that is predicted by the Debye model
Debye model
In thermodynamics and solid state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat in a solid. It treats the vibrations of the atomic lattice as phonons in a box, in contrast to the Einstein model, which treats the...
for harmonic lattice vibrations is the Mie-Grünheisen equation of state:
where
is the heat capacityHeat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
and
is the Debye gamma. The latter is one of many Grünheisen parameters that play an important role in high-pressure physics. A more realistic EOS is the Birch–Murnaghan equation of state.Interpreting seismic velocities
Inversion of seismic data give profiles of seismic velocity as a function of depth. These must still be interpreted in terms of the properties of the minerals. A very useful heuristic was discovered by Francis BirchFrancis Birch (geophysicist)
Albert Francis Birch was an American geophysicist best known for his experimental work on the properties of Earth-forming minerals at high pressure and temperature, in 1952 he published a well-known paper in the Journal of Geophysical Research ,where he demonstrated that the mantle is chiefly...
: plotting data for a large number of rocks, he found a linear relation of the compressional wave velocity
of rocks and minerals of a constant average atomic weightAtomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
with density :.This makes it possible to extrapolate known velocities for minerals at the surface to predict velocities deeper in the Earth.
Other physical properties
- ViscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
- Creep (deformation)Creep (deformation)In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
- MeltingMeltingMelting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
- Electrical conduction and other transport properties