Mineral physics

Mineral physics is the science of materials that compose the interior of planets, particularly the Earth. It overlaps with

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

Shock compression

Many of the pioneering studies in mineral physics involved explosions or projectiles that subject a sample to a shock. For a brief time interval, the sample is under pressure as the

Multi-anvil presses involve an arrangement of anvils to concentrate pressure from a press onto a sample. Typically the apparatus uses an arrangement eight cube-shaped tungsten carbide anvils to compress a ceramic octahedron containing the sample and a ceramic or Re metal furnace. The anvils are typically placed in a large hydraulic press. The method was developed by Kawai and Endo in Japan.^[2] Unlike shock compression, the pressure exerted is steady, and the sample can be heated using a furnace. Pressures of about 28 GPa (equivalent to depths of 840 km),^[3] and temperatures above 2300 °C,^[4] can be attained using WC anvils and a lanthanum chromite furnace. The apparatus is very bulky and cannot achieve pressures like those in the diamond anvil cell (below), but it can handle much larger samples that can be quenched and examined after the experiment.^[5] Recently, sintered diamond anvils have been developed for this type of press that can reach pressures of 90 GPa (2700 km depth).^[6]

Diamond anvil cell

The

Creating high temperatures

Achieving temperatures found within the interior of the earth is just as important to the study of mineral physics as creating high pressures. Several methods are used to reach these temperatures and measure them. Resistive heating is the most common and simplest to measure. The application of a voltage to a wire heats the wire and surrounding area. A large variety of heater designs are available including those that heat the entire diamond anvil cell (DAC) body and those that fit inside the body to heat the sample chamber. Temperatures below 700 °C can be reached in air due to the oxidation of diamond above this temperature. With an argon atmosphere, higher temperatures up to 1700 °C can be reached without damaging the diamonds. A tungsten resistive heater with Ar in a BX90 DAC was reported to achieve temperatures of 1400 °C.^[8]

To deduce the properties of minerals in the deep Earth, it is necessary to know how their density 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:

${\displaystyle \left({\frac {dP}{dT}}\right)={\frac {\gamma _{D}}{V}}C_{V},}$

where ${\displaystyle C_{V}}$ is the heat capacity and ${\displaystyle \gamma _{D}}$ 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.^{[9]: 66–73}

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

${\displaystyle v_{p}}$

${\displaystyle {\overline {M}}}$

${\displaystyle \rho }$

${\displaystyle v_{p}=a{\overline {M}}+b\rho }$.

This relationship became known as Birch's law. This makes it possible to extrapolate known velocities for minerals at the surface to predict velocities deeper in the Earth.

Other physical properties

• Viscosity
• Creep (deformation)
• Melting
• Electrical conduction
  and other transport properties

Methods of crystal interrogation

There are a number of experimental procedures designed to extract information from both single and powdered crystals. Some techniques can be used in a diamond anvil cell (DAC) or a multi anvil press (MAP). Some techniques are summarized in the following table.

First principles calculations

Using quantum mechanical numerical techniques, it is possible to achieve very accurate predictions of crystal's properties including structure, thermodynamic stability, elastic properties and transport properties. The limit of such calculations tends to be computing power, as computation run times of weeks or even months are not uncommon.^{[9]: 107–109}

History

The field of mineral physics was not named until the 1960s, but its origins date back at least to the early 20th century and the recognition that the

References