All of the chemical reactions discussed in this chapter have been discontinuous reactions with a fixed reaction stoichimetry and a reaction line or curve on a T-P stability diagram. When one or more of the phases involved in a chemical reaction has a variable composition, such as a magma or a solid solution mineral, the chemical reaction may be a continuous reaction. Continuous reactions have a changing reaction stoichimetry and occur over a range of temperatures at a chosen pressure. During a continuous reaction, at least one of the reactant or product phases changes composition.
Figure 22. H2O-C12H22O11 equilibrium phase diagram. Click on the image to see a larger version with more information.
A familiar example of a continuous reaction is the heating of a saturated solution of salt (brine) or sugar (syrup). If a sugar-saturated syrup (undissolved sucrose crystals are present), the equilibrium syrup composition will become more sucrose-rich, continuously, as the temperature rises. You can see this in Figure 22 on a graph of experiments on the saturated syrup composition (weight percent sucrose) as a function of temperature. Click on the diagram to see a larger version, then click the "Coordinates" button and move your mouse along the sucrose-saturation curve to see numerically how the expected syrup composition changes with increasing temperature.
Because all liquids may have variable chemical compositions, most reactions involving magmas and minerals such as melting/crystallization reactions are continuous reactions. You can see this on other temperature-composition phase diagrams such as the diopside-anorthite T-X diagram and read more about it in the chapter on Saturation Diagrams. Although there commonly are special temperatures and associated liquid compositions for which the melting/crystallization reaction is discontinuous, these typically involve an additional phase (e.g. a eutectic reaction) or a reduced number of components (e.g. ice alone melting). An example of a discontinuous melting/crystallization reaction is the ice-syrup-sucrose eutectic at -13.9°C visible on Figure 22.
For continuous reactions, the same phase may be both a reactant and a product, but with slightly different chemical compositions. When a sucrose-saturated syrup is heated, the reaction is:
sucrose + syrup (X) = syrup (X+)
where the product syrup (X+) has a higher weight percent sucrose than the reactant syrup (X). If the mineral melting is a solid solution, like plagioclase, then the mineral is both a reactant and product. The plagioclase melting reaction is:
plagioclase (AnX) + magma (AnY) = plagioclase (AnX+) + magma (AnY+)
Figure 23. NaAlSi3O8-CaAl2Si2O8 equilibrium phase diagram. Click on the image to see a larger version with more information.
