1. Unary and Binary Phase Diagrams
GLY 4200
Fall, 2016

2. Unary Diagrams Unary diagrams have a single component
We therefore usually choose to plot both P (pressure) and T (temperature) since composition is not a variable
Examples
Water
Silica
Carbon

3. Phase Rule for Unary Systems
f = c - p + 2 = 1- p + 2 = 3 - p
If one phase is present, there are two degrees of freedom (both T and P)
If two phases are present, there is one degree of freedom (either T or P)
If three phases are present, there are no degrees of freedom

4. H2O Diagram Image: h2ophase.gif
Earth is often known as the “water planet” or the “blue planet”, and there is little doubt about the importance of water in geology as a whole and in mineralogy.
Important features of the water phase diagram:
1. Within the regions labeled solid, liquid, or gas, f = 2
2. Along the boundaries between regions, f = 1
3. At the triple point, f = 0. The triple point occurs at 4.58 torr and ̊C.
4. Note vocabulary: melting/freezing, vaporization/condensation, and sublimation/deposition
5. Increasing T leads to a more disordered state
6. Increasing P leads to an increase in order
7. The critical point occurs when water ceases to have two separate fluid phases. Above the critical point, only one fluid phase exists, and the number of degrees of freedom increases to two.
8. Units – historically the torr was 1 mm of Hg. It has since evolved to be 1/760 of a standard atmosphere, which makes it slightly less than 1 mm of Hg, by %. The torr is NOT an SI unit, but the SI prefix milli is often used (millitorr). 1 millitorr = pascals. 1 Torr = ×10−3 atmospheres.
The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644

5. Terminology
Sublimation is the transition of a substance directly from the solid to the gas phase without passing through an intermediate liquid phase.
Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. The reverse process of sublimation is desublimation, or deposition.
Fusion is another name for melting.
Vaporization (or vaporisation in British English) of an element or compound is a phase transition from the liquid phase to vapor
Evaporation is a phase transition from the liquid phase to vapor ( a state of substance below critical temperature and critical pressure) that occurs at temperatures below the boiling temperature at a given pressure. Evaporation usually occurs on the surface. Evaporation may occur when the partial pressure of vapor of a substance is less than the equilibrium vapour pressure.
Boiling is a phase transition from the liquid phase to gas phase that occurs at or above the boiling temperature. Boiling, as opposed to evaporation, occurs below the surface. Boiling occurs when the equilibrium vapour pressure of the substance is greater than or equal to the environmental pressure. For this reason, boiling point varies with the pressure of the environment.Evaporation is a surface phenomenon whereas boiling is a bulk phenomenon.
Image: triple_point.gif

6. Water ice Image: ice-phase-diagram.gif
Important features of the water ice phase diagram:
1. Ice has at least 15 different phases. Each phase of ice has a different structure, and a different density.
2. Not that no more than three phases coexist at any point. However, there are numerous triple points, with f = 0.
3. As well as crystalline forms, solid water can exist in amorphous states as amorphous ice (ASW) of varying densities. Water in the interstellar medium is dominated by amorphous ice, making it likely the most common form of water in the universe. Low-density ASW (LDA), also known as hyperquenched glassy water, may be responsible for noctilucent clouds on earth and is usually formed by deposition of water vapor in cold or vacuum conditions. High density ASW (HDA) is formed by compression of ordinary ice Ih or LDA at GPa pressures. Very-high density ASW (VHDA) is HDA slightly warmed to 160K under 1–2 GPa pressures.

7. Silica Phase Diagram Image: silica-phase-diag.gif
Important features of the silica phase diagram:
1. Although quartz is by far the best known mineral, silica has a number of different mineral phases, which can serve as indicators of the P-T conditions that existed when a rock form if they are metastably retained.
2. The transition from α-quartz to β-quartz, and vice-versa, involves movement of ions, but no bond breakage, and is therefore very rapid. It is extremely unusual to find metastable β-quartz retained.
3. Coesite and stishovite are high pressure forms of silica. Stishovite is often found in meteorite impact zones, where it was created by shock metamorphism from the impact pressure wave.
4. As with previous diagrams, there are several triple points, with no degrees of freedom.
5. Tridymite and cristobolite are indicators of high-temperature mineral formation. Either may be retained metastably.

8. Snowflake Obsidian
The white patches are crystals of cristobolite, formed at high temperature
The magma-crystal mix then erupted, chilling the magma and forming a volcanic glass matrix around the metastable cristobolite, which cooled too quickly to revert to tridymite
Source:

9. Carbon Phase Diagram Image: Carbon_Phase_Diagram.gif
Important features of the carbon phase diagram:
1. There are three solid polymorphs, graphite, diamond, and solid III.
2. Solid lines are phase boundaries. Dashed lines are approximate phase boundaries, or the approximate point at which metastable diamond or graphite is converted to the other.

10. Climate Change Application
From David Archer, Global Warming, figure 7.2
The “Goldilocks” diagram – just right.
Greenhouse Warming on Terrestrial Planets