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 3.07B. Ice line added. Halite-saturated compositions for mixtures of water and halite (NaCl) are shown as a dashed halite-saturation line on a graph of weight percent NaCl vs. Temperature. Also shown is an "ice line," below which some ice will be present in the brine.
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.
The "ice line" in Figure 3.07B identifies for H2O-NaCl brines the temperatures at which crystals of ice begin to grow upon cooling. Sea water does not freeze solid at -2°C. H2O-NaCl brines do not freeze solid when their temperature falls below the "ice line." Ice crystals precipitate from the brine as the temperature is lowered below the "ice line." Ice is pure H2O and there is no solid solution between H2O and NaCl. Below the "ice line," mixtures of H2O and NaCl will be mixtures of ice (pure H2O) and brine (a saltwater solution with a weight percent NaCl that depends on the temperature). Indeed, as the temperature falls and more ice crystals grow, the brine must become saltier because H2O is being separated from it. Figure 3.08 shows 3 mixtures of ice and brine at equilibrium. Temperatures and brine composition (weight percent NaCl) are shown for each. Note that the ice crystals float on the brines, which occurs because the ice crystals have a lower density than the brines.
Figure 3.08. Ice+Brine mixtures. Mixtures of ice and brine in equilibrium at temperatures below 0°C. Click on the image to see a larger version with more information.
Based on the definition of saturation, the brines in Figure 3.08 are saturated with ice. Each of the brines is in equilibrium with ice. If ice crystals are added to any of the beakers in Figure 3.08, they will not dissolve because the solution is already saturated with ice. The dashed line giving the ice-saturated compositions in Figure 3.07B (called the "ice line" in the previous paragraph) is inclined, showing a higher weight percent H2O in the brine at saturation with ice for higher temperature mixtures and a lower weight percent H2O in the brine at saturation with ice for lower temperature mixtures. In fact, the two dashed lines in Figure 3.07B are both saturation curves: one for halite and one for ice. Notice that because both saturation curves are inclined, saturation of the brine can be caused by lowering its temperature as well as by dissolving crystals of halite or ice.
Geologists learn to extrapolate features, such as contacts, between outcrops. As a geologist, when you look at the two dashed lines in Figure 3.07B , you may imagine that the two dashed lines (the two saturation curves) will intersect if extended. They do! Go to the next page to see the resulting diagram.
