Most minerals are more soluble at higher temperatures than at lower temperatures. So the dashed line giving the halite-saturated compositions in Figures 3.04 and 3.05, should be inclined to show a higher weight percent NaCl at saturation for higher temperature mixtures and a lower weight percent NaCl at saturation for lower temperature mixtures. It is possible to do the experiments at other temperatures to determine the exact halite-saturation compositions, but the location of the saturation line on the saturation diagram should look something like that shown in Figure 3.07.
Figure 3.07. Halite saturation line is inclined. Halite-saturated compositions for mixtures of water and halite (NaCl) are shown as a dashed line on a graph of weight percent NaCl vs. Temperature. Raising the temperature increases the weight percent NaCl in brines saturated with halite. Click on the diagram a larger, interactive version for more information.
Consider now the effect of temperature on the brines that are not saturated with halite. You probably know that if pure water (0 weight percent NaCl) is cooled, crystals of ice (H2O) will grow from it (it will freeze) when the temperature drops to 0°C. You may also know that solutions that have halite dissolved in them will not freeze when the temperature drops to 0°C. Sea water, for example, which has a salinity of about 3.5 weight percent NaCl will not begin to grow crystals of ice until the temperature drops to -2°C. And brines with even higher concentrations of NaCl do not begin to freeze until even lower temperatures. Let's put a line on Figure 3.07 that shows the temperature at which crystals of ice begin to grow upon cooling. Click on Figure 3.07 to enlarge it, then click the "Show Ice Line" button on the enlarged version.
