phased

Phase diagrams are summaries of the properties of chemical systems at a variety of conditions.
Examples of phase diagrams: text Figs. 3.12, 19.24, 21.20.

A phase diagram shows how the chemical system responds to changes in the variables. The information for constructing phase diagrams comes from experiments

Variables may include:

Temperature
Pressure
Composition

Some results of experiments on silicate chemical systems:

- for mixtures the temperature of initial melting is lower that the melting temperature of any of the individual components

The diagram above summarizes experiments on a mixture of two minerals (A and B) To understand this phase diagram follow the melting history of a mixture of A and B indicated by the arrow on the right hand side of the diagram.

- Start with a mixture of about 20% B and 80% A (the bottom of the arrow). Heat this up (follow the arrow upward).

At temperature T₁ the mixture starts to melt. The composition of the liquid that forms is M. The mixture stays at temperature T₁ while B and A melt. All heat added is used in melting the solids. Eventually all the B is used up. The mixture now consists of crystal of A and liquid. As heat is added the temperature goes up. As that happens, more A melts. This moves the composition of the liquid closer to A.
Finally at T₂ all A melts. The 'system' is all liquid and that liquid has the composition of the starting material (20%B and 80%A).
Further temperature increase just makes the liquid hotter.
Any starting composition will have the same history. First melt with composition M appears at temperature T₁. If the starting composition lies between M and A, then all B will melt before A. If the starting composition lies between M and B then all A will melt before B. Finally melting of the last crystal is at the temperature at which the liquid reaches the composition of the starting material.

NOTE:
- the composition of the initial melt is usually significantly different from the bulk composition of the system.
- the temperature for complete melting is significantly higher than that at which melting begins.

Consequences for magma

- magma comes from melting rock in the earth's interior. That rock is a mixture of minerals.
- the magma usually differs markedly in bulk composition from the source material
- not all of the source is melted (no complete melting, T doesn't get high enough)

This is a diagram that shows what happens when a volume of hot rock rises from the asthenosphere to the surface. It explains melting by pressure decrease.

- Asthenosphere at a depth and temperature that correspond to the base of the arrow rises due to thinning of the overlying crust. As the rock rises its pressure goes down as indicated by the arrow. This decrease in pressure causes the rock to cross the curve at which melting begins. (This corresponds to the temperature T₁ in the diagram above.)

This melt will rise faster than the rising hot rock, and may leak up along cracks and erupt as lava at the surface.
As the hot rock continues to rise it continues to melt until it gets to a shallow level of the earth and starts to cool down (arrow path bends left). The rock cools until it again crosses the curve that marks the temperature of first melting. At temperatures lower than the ones of this curve the rock again becomes totally solid.

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