3.4 The Lever Rule

The Lever Rule is a simple graphical relationship between the proportions of phases in a mixture. If you have a good understanding of the Lever Rule, you will have a powerful tool at your disposal for the study of igneous and metamorphic rocks. Because the weight percent proportions must add up to 100, when there are only two phases (such as brine and halite) there is only one variable.

Figure 3.05. Chemical composition diagram. The proportions of saturated brine and halite mixtures to the right of the dashed line can be read from the green composition axis. Click on the graph to see a larger, interactive version with more information about the green composition axis and the lever rule.

If the weight percent of one phase is known to be X, the weight percent of the other phase must be (100-X). If the bulk composition of the mixture is known, the weight percent of each phase can be calculated. The Lever Rule works like a composition axis from 0% to 100% that is stretched from the composition of one phase to the composition of another phase in a two-phase mixture. In the case of a brine-halite mixture, the "lever" or "tie line" extends from the brine composition to the halite composition. These relationships are shown in Figure 3.05. Click on Figure 3.05 to expand the figure, then click on the Lever Rule button.

As you saw in the enlarged version of Figure 3.05, the bulk composition of a mixture of two phases divides the line between them (a tie line) into two parts whose relative lengths give the relative masses of the two phases. This relationship is called the

Figure 3.06. Lever Rule with weights. Click on the image for a larger version and to learn more about the lever rule.

Lever Rule because the mass of one phase would balance the mass of the other phase if the tie line were a lever and a fulcrum was placed at the position of the bulk composition. Figure 3.06 shows a lever and weights (circles whose area are proportional to the masses) for the brine-halite equilibrium shown in the previous figures. Click on Figure 3.06. Drag the fulcrum to change the masses and see how the Lever Rule works.

A mixture of 50g of water (H2O) and 50g of halite (NaCl) is made at 21°C and stirred until equilibrium is attained. What is the weight percent brine in this mixture (number only)?

Press "Enter" after you type in the number.