5.10 Summary

  • "A" chemical reaction is “…a process in which one or more substances, the reactants, are converted to one or more different substances, the products. …. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.” (britanica.com)
  • Evidence that chemical reactions have occurred in a rock's history can be found in its mineralogy and textures including euhedral crystals, vesicles, reaction zones, and pseudomorphs.
  • Interpretation of reaction evidence is aided by the law of mass conservation, which must be satified by any proposed reaction. The mass of the products of the reaction must equal the mass of the reactants.
  • Composition diagrams showing the minerals present in a rock can be used to help identify chemical reactions that may have occurred.
  • For a 2-component system (composition diagram is a line), conservation of mass requires that one of the phases in a discontinuous reaction must have a composition that is between the compositions of the other two, and it will be on the opposite side of the reaction.
  • For a 3-component system (composition diagram is a triangle), discontinunous reactions may be either terminal or non-terminal.
  • A terminal reaction is one in which a phase is eliminated from all equlibrium assemblages in the system. Because of the law of mass conservation, in 3-component systems the terminated phase must have a chemical composition inside the composition triangle defined by the other 3 phases in the reaction.
  • A non-terminal reaction is one in which equilbrium assemblages change, but no phase is eliminated for all bulk compositions. For non-terminal reactions in 3-component systems, none of the 4 phases lies inside a triangle defined by the other 3 phases.
  • For systems with more chemical components, composition diagrams can be reduced to lines or triangles by projecting from the composition of one or more phases that is present in all the rocks considered.
  • A much-used example of a projected composition diagram is the Thompson AFM projection from muscovite, quartz, and water, which is used for metamorphosed shales. At conditions for which muscovite is not stable but K-feldspar is present, a similar Thompson AFM projection from K-feldspar, quartz and water may be used.
  • 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. During a continuous reaction, at least one of the reactant or product phases changes composition.
  • For a continuous reaction, the same phase may be both a reactant and a product, each with slightly different chemical compositions.
  • An exchange reaction is a special type of continuous reaction in which two minerals each exchange one atom or component for another, such as Fe exchange for Mg between garnet and biotite. Exchange reactions do not change the molar proportions of the phases.