Structure, Stability and Diffusivity in Minerals.

Atomistic simulation is well suited to the calculation of the structure and relative stability of a whole range of ceramics. However, the high temperatures and pressures found within the Earth make the use of simulation even more appealing because experiments are often difficult and subject to a high degree of error.

Ionic Conductivity of a Structural Analogue of MgSiO3

MgSiO3 perovskite is thought to be the dominant phase of the Earth's lower mantle and hence accounts for approximately 40% of the earth by volume. The conductivity of the material has been investigated by experiment and simulation although the experiments are subject to large error and competing experimental groups have not been able to agree on their conclusions. An alternative is to study analogue fluorides which require much lower temperatures and lower pressures to produce the same volume contraction and to melt.

We have simulated, using molecular dynamics, the effect of temperature and pressure on the fast ionic conductivity of such an isostructural analogue; KCaF3. The effect of pressure results in a decrease in the critical temperature for conduction relative to the melting point indicating that conductivity may be possible deep within the mantle.


(3K)

In addition, animation of the trajectories of the atoms allows prediction of the diffusion mechanism. This is shown below to result from a concerted hopping motion for perovskite.


(17K)

related references:

  1. Watson G.W., Wall A. and Parker S.C.
    'A molecular dynamics simulation of the effect of high pressure on fast-ion conduction in a MgSiO3- perovskite analogue; KCaF3'
    Phys. Earth Planet. Int. 89, 137-144 (1995)

  2. Watson G.W., Parker S.C. and Wall A.
    'Molecular dynamics simulation in fluoride perovskites'
    J. Phys. Condens. Matter. 4, 2097-2108 (1992).

Pressure Induced Amorphization of Quartz.

Quartz is the stable form of SiO2 up to approximately 3 GPa and is metastable up to 15-25 GPa. At these higher pressures it becomes amorphous and the mechansim for which is not well understood.

Simulations using lattice dynamics indicate that the quartz structure becomes dynamically unstable at 21.5 GPa. Animation (shown by the Java Applet below) of this mode indicates that the channels in quartz rotate in cooperation.

Relaxation of cells which have had the ions displaced along the eigenvectors of motion for this dynamically unstable mode result in collapse of the crystal structure. Below is a picture of pure quartz and a typical amorphous cell.


Pure Quartz
(29K)


Amorphized
(49K)

related references:

  1. Watson G.W. and Parker S.C.
    'Dynamical instabilities in a-quartz and a-berlinite: A mechanism for amorphization'
    Phys. Rev. B 52, 13306-13309 (1995)

  2. Watson G.W. and Parker S.C.
    'Quartz amorphization: a dynamical instability'
    Phil. Mag. Lett. (1995) 71, 59-64

Structure and Stability of Grain Boundaries.

Real materials are generally polycrystalline and the material properties can be significantly modified by the presence of grain boundaries. When considering mantle forming minerals these boundaries become important since they provide a means for creep and ion migration, likely causes of mantle rheology.
One such mineral is Periclase (MgO) which makes up 10% of the lower mantle. Three tilt grain boundaries have been modelled using lattice dynamics. Pictures of these are given below.


(110)
1.1 Jm-2
(42K)


(210)
1.5 Jm-2
(36K)


(310)
2.0 Jm-2
(36K)

The formation energies of these boundaries increase with the decrease in the density of cross linking bonds.

Further work in this area will consider the effects of pressure and temperature upon the formation energy in order to simulate realistic mantle conditions and diffusion of ions both along and across the boundary.

related references:

  1. Harris D.J., Watson G.W. and Parker S.C.
    'Atomistic simulation of the the effect of temperature and pressure on the [001] symmetric tilt grain boundaries of MgO.'
    Phil. Mag. A, 74(2), 407-418 (1996).

  2. Harris D.J., Watson G.W. and Parker S.C.
    'The effect of temperature and pressure on the structure and stability of grain boundaries in mantle forming minerals.'
    in preparation

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