Research

Mineral physics is the study of the geological materials that compose the interior of planets (particularly the Earth) at the pressure and temperature conditions of planetary interiors. Mineral physicists employ techniques from chemistry, physics, and materials science to address mineralogical and petrological problems at extreme conditions. Mineral physics research provides constraints that enable the interpretation of geophysical observations (e.g., seismic velocities) and informs geodynamical models in terms of the properties and processes of the deep Earth. 

The Mineral Physics group at Sewanee combines experimental and computational approaches to study the constitution and evolution of planetary interiors, with a focus on the role of hydrogen in the Earth's interior.

If you could safely travel into the Earth's interior, you would find that the further you went towards the Earth's core the higher the pressure you would experience. The pressures of the Earth's interior are so high that the unit we use to describe pressure planetary pressures (gigapascals or GPa) is unfamiliar to most non-mineral physicists. One GPa is about 9,900 times atmospheric pressure.  Because pressure is just force per unit area, we could also estimate that one GPa is equivilent to the pressure of about 11 African bush elephants stacked on 1 square inch. The pressure at the center of the Earth is about 365 GPa or 4,015 elephants per square inch.

A diamond anvil cell, or DAC for short, is a handheld device used by mineral physicists to produce very high pressures. Inside a diamond anvil cell are two polished diamonds with very small flat polished tips called culets. The sample material of interest is placed between these two diamond culets inside a very small sampled chamber filled with a pressure transmitting medium (Ne, Kbr, Ar, etc), and the diamond anvil cell is used to force the diamonds together which in turn pressurizes the sample. 

Because pressure is a function of area, the samples used in DAC experiments are very small—typically 10-50 micrometers or roughly half the diameter of a human hair. DACs can compress these tiny samples to the pressures exceeding those the Earth's core, although the higher the pressure the smaller the sample has to be. Diamonds are used to produce high pressures because of their hardness and transparency, and as an added bonus their high thermal conductivity aids in producing the high temperatures needed to replicate the pressure-temperature conditions of planetary interior as described below.

Achieving temperatures comparable to those within the deep Earth is just as important to the study of mineral physics as creating high pressures. Just as pressure increases towards the Earth's core, so does temperature. Although there is still debate about the exact temperature at the center of the Earth, estimates range from 4,400 to 6,000° Celsius (7,952° to 10,800° Fahrenheit). For comparison, the surface of the sun is about 5,500° C.

Our group uses two methods to achieve high temperatures:

(1) Resistive heating using small heaters created to fit inside a pressurized diamond anvil cell and have been succussfully used by our group to steadily heat samples to temperatures up to 750°C. We have combined in situ resistive heating experiments with both single crystal X-ray diffraction and Fourier transform infrared spectroscopy measurements (shown at top left). 

(2) Laser heating of samples within diamond-anvil cells can achieve temperatures above 6000 K. Our group has used offline laser heating ex situ, for "cook-and-look" reaction experiments (shown at bottom left), and in situ, using the laser capabilities of synchrotron beamlines to probe minerals at concurrent high pressures and temperatures. For more information about synchrotron radiation, click on the Links tab above.

While our previous discussion of pressure and temperature have focused on reproducing these conditions experimentally, it is also possible to calculate the stability and properties of geological materials at extreme conditions using an approach called density functional theory (DFT).

DFT calculations are a form of computational quantum mechanical modeling by which the stability and properties of minerals can be calculated based on the electron density of their constituent atoms. DFT calculations are a powerful tool to probe complex compositions, to evaluate phases at pressures beyond what is feasible in a lab, and to look for properties that are sometimes difficult to probe using experiments (e.g., elasticity). 

Our group performs DFT calculations using Quantum Espresso to assess the stability and properties of hydrous phases at high pressures, building a theoretical framework for our experiments.