7.4 Ternary Liquidus Minimum

The lowest melting composition for some solid solution minerals is at an intermediate composition. This is discussed for the alkali feldspar binary system in the Binary Solid Solution chapter. Figure 7.06 shows the liquidus diagram for the alkali feldspars at room pressure. The liquidus and solidus curves meet at a binary minimum. The ternary system (NaAlSiO₄)-(SiO₂)-(KAlSiO₄) includes the alkali feldpar binary and is shown in Figure 7.07 above. Click on the diagram to see a larger, interactive version.

If you move the slider on the larger version of Figure 7.07, you will see isothermal equilibrium sections at several temperatures. These sections may help you see that during crystallization, liquid compositions move away from the compositions of the crystallizing minerals with the possibility of reaching one of two low points on the liquidus surface. For bulk compositions between the alkali feldspar (Afs) line and the nepheline (Nph)-kalsilite (Kls) line, the last possible liquid composition is at a ternary eutectic where three cotectic curves meet, similar to those discussed in the Ternary Eutectic Diagrams chapter. For bulk compositions between the alkali feldspar line and silica corner, the last possible liquid composition is at a ternary minimum where two cotectic lines meet. However, because the crystallizing minerals are solid solutions, the last liquid to crystallize might not be at either of the low points. The last liquid to crystallize might be the cotectic liquid when the two-solids-edge of a cotectic triangle crosses the bulk composition as temperature falls. This will be true for nearly all the bulk compositions on the silica side of the alkali feldspar line.

More text coming.....    next page