5.9 Continuous Reactions (continued 😊)

Reactions among minerals in metamorphic rocks can also be continuous. Good examples can be found in metamorphosed shales where a number of the metamorphic minerals are solid solutions, especially in terms of Mg/Fe content. Figure 5.24 is a Thompson AFM diagram for metamorphosed shales that is animated to show the details of some continuous reactions as temperature is increased. Click on the diagram to view the larger, interactive version. Then move the slider to examine the mineral assemblage for the red dot bulk composition change due to a continuous chemical reaction as temperature changes. The mode at each temperature is shown numerically and graphically by the colored rectangles for each mineral present, including the projection minerals muscovite (Ms) and quartz (Qz). The Mg/Fe compositions of garnet (Grt), biotite (Bt), and chlorite (Chl) are shown, if the mineral is present, as the molar ratio Mg/(Mg+Fe), also known as the Mg#.

For some temperatures, the mineral assemblage for the red dot composition consists of two mineral (plus the projection minerals). Notice that the compositions of the two minerals both change continuously as the temperature is changed, although their proportions are nearly constant. For example, above 602°C, the mineral assemblage is Grt-Bt. As the temperature is increased, the Grt composition becomes more Mg-rich and the Bt composition becomes more Fe-rich. This continuous reaction may be written:

garnet (X_Mg) + biotite (Y_Mg) = garnet (X_Mg+) + biotite (Y_Mg-)

where X_Mg and Y_Mg stand for the molar ratio Mg/(Mg+Fe) in the garnet and the biotite. This is an Mg = Fe exchange between garnet and biotite, changing their compositions but not their mode. A convient way to write an exchange reaction like this is to use the formulas for the mineral end member compositions. For Mg = Fe exchange between garnet and biotite, the reaction is:

Fe₃Al₂(Si₃O₁₂) + KMg₃(AlSi₃O₁₀)(OH)₂ = Mg₃Al₂(Si₃O₁₂) + KFe₃(AlSi₃O₁₀)(OH)₂

(Note. For the bulk composition in Figure 5.24 the mode does change by a very small amount because the biotite changes it's Al content, too, involving Ms and Qz in the reaction.)

For temperatures between 559°C and 602°C, the red dot composition has three minerals present at equilibrium (Grt, Bt, and Chl) plus the projection minerals. Notice that the compositions and the modal percentages of the three minerals all change continuously as the temperature is changed. As the temperature is increased, the chlorite is consumed to produce garnet and some more biotite. Some muscovite is also consumed and some quartz is produced, changing their modal percentage (water, too). As the reaction proceeds, the Grt, Bt, and Chl also change equilibrium composition as well as mode.

The changes in mineral composition and mode required to maintain equilibrium as temperature and pressure change during metamorphism are complex, and there is evidence in many rocks (e.g compositionally-zoned minerals) that equilibrium is not fully achieved. However, this lack of complete equilibrium provides evidence that can be used to decifer details of the changes in temperature and pressure experienced by the rock. Equilibrium reactions, reaction textures, and reaction kinetics all must be used by petrologists to interpret the workings of the earth.

Summary