CH7 - Defects in Minerals

Question 1

What determines stability of a solid solution?

Answer 1

If the increase in TdS outweighs any increase in enthalpy due to substitution, solid solution will be thermodynamically stable

Question 2

How can defects impact mineral properties at room temp?

Answer 2

If a material is heated, many defects are created to achieve equilibrium defect concentration, then the material is rapidly cooled (quenched), the defects become frozen.

Question 3

Why does diffusion have a lower activation energy at low temps?

Answer 3

This is because at low temps the number of intrinsic defects is low and diffusion is due to extrinsic impurities. As temps increase, intrinsic defects increase exponentially while extrinsic are fixed

Question 4

What is mineral deformation controlled by?

Answer 4

Entirely by the way line defects (dislocations, 1D) are generated and move through a crystal

Question 5

What is the concept of dislocations?

Answer 5

Accounts for discrepancy between theoretical stress required for slip step, and those values found experimentally
- Narrow line defect where atoms stop - dangling bonds
- During stress dangling bonds moved from one plane to the next
- Only those atoms around dislocation line are disturbed
Most energetically favourable to become a ring

Question 6

When do stacking faults occur?

Answer 6

Occur when the periodicity of different layers is incorrect, in an isolated region of a crystal. This may be during growth of the crystal, structural deformation from one polytype to another or during deformation by glide.

Question 7

What will happen to the enthalpy of a mineral crystal with increasing defects?

Answer 7

Increase due to bond disruption, strain energy and lattice distortion, and formation energy. It is favourable to have some defects in the system, but at some point it will become too high.

Question 8

What will happen to entropy of the crystal with increasing defects?

Answer 8

Increases due to increasing configurational disorder and charge disorder as defects interact. Slightly defected is most stable - too many too broken.

Question 9

Do we expect defects can be stable within a crystal?

Answer 9

Yes but only a small amount. At high temperatures defects can increase stability through increased microstates and lattice strain relief as atoms have more movement freedom.
At low temperatures defects formed at high temps can become frozen in a metastable state. Ea too large.