Figure 5.09. KCl-NaCl Diagram. This halide system is similar to the alkali feldspars in that there is complete solid solution between the KCl and NaCl phases at elevated temperatures. Click on the diagram to view a larger version with more information.
Figure 5.10. NaI-RbI Diagram. Phases at equilibrium for the chemical system NaI-RbI are shown as a function of chemical composition (mole percent RbI) and temperature. Click on the diagram to view a larger version with more information.
Figures 5.09 and 5.10 show alkali halide phase diagrams that are similar to the alkali feldspar phase diagram. Figure 5.09 shows the melting relations of sylvite (KCl)-halite (NaCl) solid solutions. This solid solution has an azeotropic melting point and also a solvus. Because nucleation and growth of sylvite-halite compositions is comparatively rapid, it can be challenging to quench intermediate sylvite-halite composition crystals or to create perthite-like intergrowths Brady(1997).
Figure 5.10 shows the melting relations of NaI-RbI solid solutions. Even at room pressure, the NaI-RbI liquidus intercepts the solvus creating a solid solution eutectic similar to the eutectic for alkali feldspars at high PH2O. Solid solution in NaI-RbI is limited by the solidus at high temperatures and by the solvus at lower temperatures. The questions on the next page are based on this phase diagram. Test your understanding by answering the questions.
