Two-component
diagrams - Presented in two dimensions.
Understanding two component systems requires holding one degree of freedom (variable) constant.
Often pressure is held constant (isobaric) and T-X and are varied.
When two components can mix on an atomic scale
then they are considered miscible phases.
example is water and alcohol (liquid) or forsterite-fayalyite (olivine series solid-solution).
Olivine Series is an example of solid-solution series that is maintained over regardless of temperature (also includes high temperture portion of the plagioclase series).
Points to make:
1. Liquidus Diagram
2. liquidus - upper portion where melt is in equilbrium with crystalline phase.
3. solidus - lower portion where crystalline phase is in equilbrium with melt.
4. Path depicts the evolution of melt and crystaline phase composition.
5. Note: that this is scenerio operates assuming constant re-equilibration and a closed system.
6. subsolidus - region below solidus where all solid-state transformations may take place.
Example from lab exercise: Core: Mg1.63 Fe0.36 Rim: Mg1.01 Fe0.78
Shows us that equilbrium does not exist. Tells us about cooling rate.
Coexistance without mixing is known as immisciblity.
Random mixing Ordered
mixing
Unmixing
At high temperature - 1. structure expands, 2. Vibration of atoms larger, 3. distinct structural sites become similar.
Consequence is that cation os different size can occuppy the same site.
e.g., Na+ (1.18) and K+ (1.33) in feldspars
Exsolution - As tempertures decrease, diffusion of atoms within crystal occurs. Inital solid-solutions undergo ÒunmixingÓ into two separate, distinct crystalline phases.
Analogy: Vigourously shaken oil and water mixture will mix for a short time. With time they will ÒunmixÓ
See figure 5.21 (page 238). Subsolidus - phase diagram.
Define:
1. Miscibility gap - region in T-X space where solid-solution compositions are not formed due to exsolution.
2. Phase changes take place in the subsolidus
Examples: hornblende - grunerite, Labradorite An47-An58
No solid-solution
Silica* - albite system
* Silica as quartz never crystallizes from solution in water free systems (crystobilite and tridymite). But, for the purpose of our discussion (i.e., a binary system) we will ignore the third component water.
1. Imagine a melt containing the ions Na+, Al3+, Si4+, O2- with equal amounts of Na and Al and excess Si and enough O to keep things electrically neutral.
2. Above the melting temperature vigorous particle motion keeps forming and breaking of the Si-O bonds and Al and Na prevent the Si-O bonds from firmly locking into bonds.
3. As the temperature cools Si-O bonds lock and SiO2 crystals start forming.
4. The growth of SiO2 crystals change the composition of the remaining liquid. (i.e., the liquid is impoverished in Si and enriched in Al and Na).
5. If the temperature does not drop any more, then no additional SiO2 crystals will form.
6. The process does not occur in steps, but is continuous. Therefore, in the figure above there is a line from X1 to X2.
7. The line becomes steeper as crystallization proceeds.
8. A similar crystallization process occurs for albite in melts that are enriched in Al and Na.
9. The complete diagram is shown as
10. The point Z is point in T-X space where both crystal will grow. This point is known as the eutectic point.
11. The curve that represents the loci of points between melt and solid regions (i.e., phase fields) is termed the liquidus.
12. The arrows on the boundaries of the phase fields represent the crystallization paths with decreasing temperature.
13. Note that this diagram represent isobaric conditions.
14. The phase relations will change with changing pressure. Lets consider a system in the state at point D in the diagram below.
15. At a low pressure the system will exist as a single phase (liquid) state.
16. If the pressure is raised, then two phases can coexist (liquid and silica) and crystallization will occur.
Caveats -
17. The effect of changing pressure (increased or decreased) can cause melting or crystallization. The effect is dependent upon all components in the system.
18. In an ideal binary system, usually the effect of increased pressure will cause crystallization (at the same temperature).
19. The presence of water (a third component) is likely to significantly alter the behavior of a system.