8.4.1: Different Kinds of Reactions

Last updated
Save as PDF

Page ID: 18618

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

As discussed in Chapter 4, under any pressure and temperature, the most stable mineral assemblage is the one with the lowest Gibbs free energy. So, when a rock is heated or squeezed, chemical reactions occur that may consume old minerals and create new ones. These reactions may be of several types. The table seen here gives examples of different types of metamorphic reactions. By convention, the low-temperature mineral or assemblage is to the left of the equal sign; the high-temperature products are to the right.

Solid-solid reactions involve no H₂O, CO₂, or other vapor phase. The first example of a solid-solid reaction contains only two minerals, both Al₂SiO₅ polymorphs. This reaction may occur when a metamorphosed shale is heated to high temperature. But most metamorphic reactions involve more than two minerals, and many involve H₂O or CO₂. The second solid-solid reaction is more typical and involves four minerals.

Examples of Metamorphic Reactions

Solid-solid reactions:

xxxandalusite = sillimanite
xxxAl₂SiO₅ = Al₂SiO₅

xxxgrossular + quartz = anorthite + 2 wollastonite
xxxCa₃Al₂Si₃O₁₂ + SiO₂ = CaAl₂Si₂O₈ + 2 CaSiO₃

Dehydration reactions:

xxxmuscovite + quartz = K-feldspar + sillimanite + vapor
xxxKAl₂(AlSi₃)O₁₀(OH)₂ + SiO₂ = KAlSi₃O₈ + Al₂SiO₅ + H₂O

xxxkaolinite + 2 quartz = pyrophyllite + vapor
xxxAl₂Si₂O₅(OH)₄ + 2 SiO₂ = Al₂Si₄O₁₀(OH)₂ + H₂O

Hydration reaction:

xxxenstatite + 2 H₂O = 2 brucite + 2 quartz
xxxMg₂Si₂O₆ + 2 H₂O = 2 Mg(OH)₂ + 2 SiO₂

Carbonation reaction:

xxxforsterite + 2 CO₂ = 2 magnesite + quartz
xxxMg₂SiO₄ + 2 CO₂ = 2 MgCO₃ + SiO₂

Dehydration reactions and decarbonation reactions, such as the examples in this table, liberate H₂O and CO₂, respectively. Hydration reactions and carbonation reactions consume H₂O and CO₂, respectively.

Metamorphic reactions involve changes in mineralogy or in mineral composition. A mineral assemblage is at chemical equilibrium if no such changes are occurring. If the assemblage has the lowest Gibbs free energy possible for the given conditions, it is at stable equilibrium. In principle, all rocks tend toward stable equilibrium. Whether they reach it depends on many things, including temperature, grain size, and reaction kinetics. If reactions cease before a rock has reached stable equilibrium, the rock is at metastable equilibrium. Many metamorphic rocks contain metastable minerals.

We call a stable mineral assemblage representative of a given set of pressure-temperature conditions a paragenesis. When conditions change, metamorphic reactions may create a new paragenesis as some minerals disappear and others grow. Such reactions may be prograde or retrograde. Most of the reactions in the table above are prograde, but the two examples of carbonation and hydration reactions are retrograde reactions (involving orignial high-temperatue minerals reacting to form low-temperature minerals) that often affect mafic rocks.

Prograde metamorphism involves the breakdown of minerals stable at lower temperature to form minerals stable at higher temperature. Some prograde reactions are solid-solid reactions, but most involve the release of H₂O or CO₂ that flow along cracks or grain boundaries. As temperature increases, minerals containing H₂O or CO₂ become increasingly unstable, causing dehydration or decarbonation, and the release of H₂O or CO₂ as intergranular fluid. If we ignore H₂O and CO₂, we find that most prograde metamorphism is nearly isochemical, meaning that the rock is the same composition before and after metamorphism. Sometimes, however, flowing fluids and metasomatism can be the dominant forces controlling metamorphism.

Retrograde metamorphism is, in many ways, just the opposite of prograde metamorphism. Typically, H₂O- and CO₂-free minerals react with fluids to produce hydrous or carbonate minerals. Mg-silicates such as forsterite (Mg₂SiO₄), and enstatite (Mg₂Si₂O₆), for example, may react to form talc or serpentine (both hydrated Mg-silicates), brucite (Mg hydroxide), or magnesite (Mg carbonate), at low temperature. In contrast with prograde reactions, retrograde reactions are often quite sluggish. They may not go to completion and frequently do not reach stable equilibrium. Sometimes retrogression only affects parts of a rock or parts of some grains in a rock.