In the Binary Saturation Diagrams chapter, it was shown that the proportions of two phases at equilibrium could be determined by the position of the bulk composition along a tie line between the two phases, as if there is a composition axis along the tie line. An analogous lever rule works for three-phase assemblages and ternary phase diagrams. In this case, the ternary lever rule works as if a ternary graph paper grid is placed on the phase diagram with its corners at the composition of the three phases.
Figure 6.02. XYZ Barycentric Diagram. Ternary diagram with gridline. Click on the diagram to see a larger version with ternary lever rule animation.
Figure 6.02 is a ternary diagram with gridlines for plotting the proportions of the three components. Click on the diagram to see a larger version with a "Coordinates" button like the one in Figure 6.01. There is also a "Show Phase %" button in Figure 6.02. Click on the "Show Phase %" button and mouseover the diagram to see colored triangles that have areas that are proportional to the weight percentages of minerals whose compositions occur at the corners of the triangle. As you can see, the colored triangles are redundant if the grid is present, but they will be used to show ternary lever proportions on several other diagrams on this website.
Figure 6.03. H2O-NaCl-KCl Diagram. Ternary phase diagram of Figure 6/01 with a ternary lever rule grid superimposed on the brine + halite + sylvite three-phase region. Click on the diagram to see a larger version with the ternary lever rule animated by the "Show Phase %" button.
Figure 6.03 shows again the H2O-NaCl-KCl diagram of Figure 6.01, but this time with a ternary lever rule grid superimposed on the brine + halite + sylvite three-phase region. Click on the diagram to see a larger version with the ternary lever rule animated by the "Show Phase %" button. The ternary grid has been distorted from an equilateral triangle to one that fits the less regular triangle that connects the compositions of halite and sylvite to the doubly-saturated brine com position. As you would expect from the ternary grid, bulk compositions closer to one corner of the three-phase triangle, should have more of the phase at that corner.
