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Mineral Sciences

crist900

Configuration of the disordered phase of cristobalite obtained by Reverse Monte Carlo modelling of neutron total scattering data. Similar pictures are obtained using molecular dynamics simulations.

quartz_phases

The two phases of quartz, showing the rotations of the SiO4 tetrahedra that lead to changes in symmetry without a large-scale rearrangement of the structure. Quartz serves as a prototype system for testing many ideas, such as our Rigid Unit Mode theory.

garnet

Crystal structure of garnet. The yellow spheres represent the atoms on the dodecahedral sites. We have studied solid solutions with chemical variation on these sites. The important interactions between these sites are in the horizontal (and symmetrically related) directions, and involve distortions of the intermediate SiO4 tetrahedra (darker blue polyhedra).

domains

Needle domains in CaTiO3 perovskite. The origin of the needle shapes has been studied using theory and computational tools.

mole_on_pyroph

Adsorption of a molecule of carbon tetrachloride onto the later surface of pyrophyllite, as calculated using quantum mechanics methods.

brucite

Crystal structure of brucite, Mg(OH)2, which we have studied using neutron powder diffraction at high pressures and temperatures simultaneously (yellow represents Mg, red represents O and pink represents H). We have found that the best agreement with data is obtained when modelling the hydrogen atoms using a distribution of positions.

zeolite_sem

SEM image of a zeolitized volcanic glass shard. Well-formed crystals of clinoptilolite have grown from the clay coated rim of the shard inwards towards its centre.

stishovite_cij

(A) Calculated variation of the elastic constants of stishovite as a function of pressure. The large variation arises from the coupling with the order parameter.

quartz_cij

(B) Comparison of the calculated temperature-dependence of the elastic constants of quartz with experimental data. The theory provides an understanding of the elastic softening associated with the phase transition.

cristobalite_phases.jpg

The two phases of cristobalite at ambient pressure. The phase transition involves rotations of the tetrahedra, as seen in this projection of the structures of the two phases. The high-temperature cubic phase has many low-energy rigid unit modes, which allow the tetrahedra to have disordered orientations to avoid formation of the linear Si–O–Si bonds seen in this average structure.

landau_equations.jpg

Representative Landau free energy function with coupling between all elastic degrees of freedom and the order parameter Q. Minimisation gives equations for the variation of temperature of the elastic constants through their coupling with the order parameter.

quartz_domain_wall.jpg

Domain wall in quartz generated by molecular dynamics simulations.

atomsbanner.gif

 
leucite.gif

The mineral leucite: experimental studies of the phase transitions carried out by the Cambridge Mineral Sciences group have been supported by empirical modeling calculations in order to determine the transition mechanism and the energetics of Al/Si ordering.

pbi2.gif

Calculated electron density of the atoms in lead iodide, showing the covalent bonding as shared electrons.

network.gif

The low-temperature phase of tridymite – we are using molecular dynamics simulations to understand the mechanisms of the phase transition and the nature of the high-temperature phases.

graph.gif

Calculated entropy of the garnet–grossular solid solution computed using the technique of thermodynamic integration in Monte Carlo simulations.

rmcdiffuse.gif

Diffuse scattering in the [100] section of reciprocal space of b-cristobalite calculated in an RMC simulation. The structure of diffuse scattering exactly matches electron diffraction maps, and is consistent with the predictions of the Rigid Unit Mode model.

splitatom.gif

The split-atom representation of a linkage between two rigid tetrahedra. This representation provides an algorithm for calculating the complete set of RUMs of any network structure.