Conservation of Mass

An important tool for identifying the reaction(s) that may have occurred is the Law of Conservation of Mass. If you add up the masses of all the products in a chemical reaction, the total must equal the sum of all the masses of the reactants. Minerals can be created by chemical reactions, but atoms cannot. For example, the sum of all the silicon atoms in the products must equal the sum of all the silicon atoms in the reactants. This feature helps us write possible chemical reactions.

Similarly, the mass of any chemical component in the reactant phases can be redistributed, but sum of the masses of that chemical component in the reactant phases must equal the sum of the masses of that chemical component in the product phases. This feature helps us identify possible reactions using composition diagrams.

Recognizing Reactions on Composition Diagrams

Even if there is not good textural evidence for chemical reactions, a comparison of the mineral assemblages in two rocks of similar composition can help identify possible reactions that relate the two rocks. For 1-component systems (all phases plot at a single point), there is little choice with only 2 phases in possible reactions. Both the single reactant phase and the single product phase (e.g. kyanite and sillimanite) have the same composition. For chemical systems with more than one component, mineral assemblage diagrams can be very helpful in understanding chemical reactions.

For 2-component systems (all phases can be plotted on a line), 3 phases are involved in each possible reaction. Conservation of mass requires that one of the phases in the reaction must have a composition that is between the compositions of the other two and will be on the opposite side of the reaction.

Reaction lines for two chemical reactions in the 2-component system NaAlSiO4-SiO2 are shown on a T-P diagram in Figure 10. Click on the diagram to try your hand at deducing the two reactions from the linear composition graphs. In a 2-component system, a phase is either introduced or eliminated (terminal reaction), depending on direction the reaction is crossed. A particular rock might or might not be affected by a reaction, depending on the bulk composition. You can see both of these features in Figure 11 after you answer the reaction question. Be sure to use the "Show MAD" button to see a bulk-composition-specific T-P stability diagram.

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