Figure 4.09 summarizes the equilibrium relations of the phases brine, halite, and ice that have been discussed in this chapter. It is, therefore, a phase diagram. 9
Figure 4.09. H2O-NaCl equilibrium phase diagram. Phases at equilibrium for the system H2O-NaCl are shown as a function of chemical composition (weight percent NaCl) and temperature. The blue area indicates temperatures and compositions for which a single phase, brine, is present at equilibrium. Click on the diagram to enlarge and to get more information.
The horizontal line patterns represent tie lines that connect the compositions of pairs of phases at equilibrium at specific temperatures. For the "Brine+Halite" area, the brine composition at each temperature is given by the halite-saturation curve. For the "Ice+Brine" area, the brine composition at each temperature is given by the ice-saturation curve. Notice that both halite and ice have no variation in chemical composition. This is because there is no solid solution of H2O in halite and there is no solid solution of NaCl in ice.
As predicted, the halite-saturation curve and the ice-saturation curve meet. The brine composition at the intersection point (23.20 weight percent NaCl) is for a liquid that is saturated both with halite and with ice. If ice crystals, halite crystals, and brine are all present at equilibrium, (1) the brine must be saturated both with halite and with ice, (2) the brine composition must be 23.20 weight percent NaCl, and (3) the temperature must be -21.21°C. This is a special, fixed point that can be reproduced in any laboratory by mixing halite and ice with water and stirring until equilibrium is reached. It is the temperature produced in the tub of an old-style ice cream maker when ice and halite are mixed in excess. It is also the temperature below which rock salt (halite) will not react with (melt) snow and ice. Below -21.21°C, ice and halite can be in contact at equilibrium. "Ice+Halite" is the third two-phase equilibrium area on Figure 4.09.
We are about to look at diagrams like Figure 4.09 for other minerals and higher temperatures, and use them to understand igneous processes. Let's make sure that you have a good understanding of the ice-brine-halite diagram. Use the enlarged version of Figure 4.09 and the "Coordinates" button (if needed) to help answer the following questions.
What what phase or phases will coexist at equilibrium for a chemical composition of 10 weight percent NaCl at -10°C?
Yes, the point for 10 weight percent NaCl and -10°C plots in the 'Ice+Brine" field. No, the point for 10 weight percent NaCl and -10°C plots in the 'Ice+Brine" field. To see what phases will be present at equilibrium, plot the composition and temperature on the phase diagram and read the label on the diagram where the values plot.
Yes, the ice-saturated brine at -10°C would have a composition of 14 weight percent NaCl. No, the ice-saturated brine at -10°C would have a composition of 14 weight percent NaCl. You can find this value by using the "Coordinates" button in the enlarged version of Figure 4.09. Look for the composition of the ice-saturation curve at -10°C.
Press "Enter" after you type in the number.
Good thinking. All mixtures of ice and halite will begin to melt at -21.21°C. The formation of liquid, in this case brine, is due to a reaction between the ice and the halite in the proportions 23.2 grams of halite to 76.8 grams of ice.
No. All mixtures of ice and halite will begin to melt at -21.21°C. The formation of liquid, in this case brine, is due to a reaction between the ice and the halite in the proportions 23.2 grams of halite to 76.8 grams of ice. Use the phase diagram. To determine the temperature of first melt, find the composition of the rock, then follow that composition up in temperature until it enters a field that contains liquid. In this case, all proportions of ice to halite (all compositions) yield the first liquid at -21.21C.
