3.8 Summary

  • Temperature vs. chemical compositon diagrams can be used to summarize experimental data on the melting and crystallization of two-component chemical systems.
  • Saturation curves on these diagrams identify the chemical composition and temperature of liquids saturated with a crystal.
  • In general, saturation curves show that the solubility of a mineral increases as temperature increases.
  • The proportions of liquid and crystal for a saturated solution of a particular bulk composition can be determined graphically from the saturation curve diagram using the lever rule.
  • The lever rule works like a composition axis fitted to the tie line between the crystal composition and the saturated liquid composition.
  • Halite and ice react to form a liquid (melting occurs) at temperatures below the melting temperature of either mineral alone.
  • Saturation curves for ice and halite meet at -21.21°C and 23.30 weight percent NaCl, a special point (eutectic) that is the minimum temperature that NaCl brines can occur at 1 atmosphere pressure.