Lecture 11 - Exsolutions Flashcards
(10 cards)
Why does exsolution occur?
Solid solutions are frequently not compatible at low temperatures. This is because at high temperatures, the differences in atomic radius can be easily accommodated as the atoms are highly thermally agitated. Upon cooling unsustainable lattice deformations occur
Thus, minerals often segregate into discrete phases i.e. exsolve
What is spinodal decomposition?
A method of exsolution whereby a solid solution can unmix into regions of different composition in a continuous process
Only works for materials where there is structural similarity between initial and final phases (because structure does not really change, only chemistry with slight deformation)
How can energy be lowered by exsolution?
By nucleation if a major compositional change is needed. For unstable phases, energy can be lowered by continuous fluctuations and can unmix by spinodal decomposition
Is nucleation or spinodal decomposition faster for lowering energy?
Spinodal is always faster at the same temperature because there is no dG, only a small kinetic barrier
Where can spinodal decomposition be seen in geology?
Volcanic rocks where there is a fast cooling rate, and in geological settings with low temperatures and slow diffusion rates over time scales
Where does interfacial energy come from?
- Surface energy
- Misfit strain
Both temperature independent
Decrease in surface roughness = decrease in interfacial energy and strain energy
What controls the periodicity of the unmixing lamellae?
Lamellae that form during exsolution typically coarsen with time. The periodicity is described by wavelength and exsolution spacing is described by kinetics (relationship between rates and concentration with time).
Wavelength dictated by strain (different unit cell sizes) and minimization of gradient energy (limit interactions between two compositions as these are high energy)
Are exsolution and spinodal decomposition diffusion processes?
Yes. These solid state processes involve the unmixing of two elements to form more stable phases and during cooling this is heavily reliant on diffusion (diffusion results from thermal oscillations which allow atoms to jump)
What is the impact of slow cooling on exsolution evolution?
Sharp interface between phases is produced from the beginning. Nucleation can only occur where there is supersaturation is reached, this is dependent on undercooling. Slow cooling will result in few nucleation events and a long diffusion distance.
What is the impact of fast cooling on exsolution evolution?
Fast cooling will allow for faster nucleation due to more undercooling. This results in less space between each nuclei. For every 2 orders of magnitude difference in cooling rate there is a change of 1 order of magnitude in terms of microstructure.
