# Multicomponent phase diagrams

## Two-component, liquid/liquid phase diagrams

 Since plagioclase is one of the most common minerals in the earth's crust, we will discuss the phase diagram for the plagioclase system. The phase relations in the plagioclase system are shown in Figure 3 at constant pressure equal to that of the atmosphere (atmospheric pressure is 1 bar). In Figure 3 the upper curve is called the liquidus and the lower curve is called the solidus. At temperatures above the liquidus everything is liquid, below the solidus everything is solid (crystals of plagioclase solid solution). At temperatures between the solidus and liquidus crystals of plagioclase solid solution coexist in equilibrium with liquid. www.tulane.edu/~sanelson/images/phdifig4.gif www.tulane.edu/~sanelson/images/phdifig4.gif aPartial miscibility in binary systems at a pressure of 1 bar. The horizontal lines are the Tie lines. (a) water/phenol. Data by Hill and Malisoff; (b) n-hexane/aniline. Data by Keynes and Hildbrand; Koningsveld, Fig. 38, p. 47. Keynes and Hildbrand; Koningsveld, Fig. 38, p. 47.

## Two-component, liquid/solid phase diagrams

www.tulane.edu/.../geol212/2compphasdiag.html

How to Read a Phase Diagram: This two-phase diagram features two types of curves. Lines separating partially liquid phases from fully liquid phases are known as liquidis lines while the line separating partially melted phases from solids are known as solidus. The point where all three lines meet (point E in this diagram) is known as the eutectic point. Near the eutectic point, a samples behavior is very dependent on its composition. For example, wetting behavior may differ dramatically between the situation of solid a wetting with liquid b vs solid b wetting with liquid a.

The horizontal axis shows the relative composition between the two phases while the vertical axis indicates the temperature of the system. Melting far from the eutectic point may occur over a range of temperatures between solidus and liquidus where both phases are fully melted.

If one has a mixture of composition X at temperature [itex]T_2[/itex], the percent of A in solid form would be given as the following: % solid A = b/(a+b)*100. The remaining portion of A in the mixture is melted.

## Three-component phase diagrams

Koningsveld, Fig. 86, p. 83.
Koningsveld, Fig. 100, p. 95.