Different Kinds of Reactions

Dave Hirsch, Western Washington University

Examples of phase diagrams and reactions in jpg, pdf and animated-pdf format are available.
A 5 page summary (Acrobat (PDF) 158kB Aug1 07) of this information is available, which can be used as a class handout.

Reactions among solid and fluid phases can be categorized in several different ways, based on what the reaction does, how the reaction progresses, or based on the nature of phases involved.

Categorization based on reaction effect

  • Net-transfer reactions involve chemical components being "transferred" from one phase or set of phases to others (new phases are produced as old ones disappear). An example is:
    anorthite = grossular + kyanite + quartz
    Net-transfer reactions may be terminal reactions or tie-line flip reactions (discussed below).
  • Exchange reactions involve chemical components being exchanged between phases, so compositions change, but modes remain the same (no phases disappear and no new phases are produced). An example is:
    Fe (in garnet) + Mg (in biotite) = Mg (in garnet) + Fe (in biotite)

Categorization based on reaction progress

  • Discontinuous reactions are those that occur at a particular temperature (for a particular pressure). For these, a curve (or line) can be drawn on a pressure-temperature (P-T) diagram. (See the kyanite=sillimanite reaction on the phase diagram shown below.) On either side of the curve, a different set of phases is stable. In a discontinuous reaction, products and reactants can only co-exist stably precisely at the equilibrium reaction conditions (on the reaction line in P-T space). Discontinuous reactions are always net-transfer reactions.
  • Continuous reactions involve phases that may vary in composition. Such reactions are more common than discontinuous reactions because compositional variation, of either fluid or solid phases, is typical for geological materials. Continuous reactions occur over a range of conditions, so the products and reactants coexist stably over a range of conditions (but the compositions of the phases changes systematically as conditions change). Continuous reactions may be net-transfer reactions or exchange reactions.

Categorization based on phases involved

  • Solid-solid net transfer reactions (often just called solid-solid reactions) occur among solid phases of differing composition. These phases may include elements found in fluids (H, C), but those elements are conserved in the solid phases so that no fluid phases (H2O, CO2) are involved as reactants or products. Solid-solid net transfer reactions can be continuous or discontinuous, and they may be terminal, or tie-line flip reactions (discussed below).
  • Polymorphic reactions are a special type of solid-solid reaction that involves phases of identical composition. Classic examples are the reactions among the aluminum silicates (kyanite-sillimanite-andalusite; see the photomicrograph and phase diagram shown at right), the conversion of graphite to diamond at high pressure, and calcium carbonate (calcite-aragonite) equilibria.

  • Most solid-solid reactions plot on a P-T diagram as essentially straight lines. This is because the ΔS and ΔV of reaction do not change much with varying pressure or temperature. To the extent that ΔS and ΔV do change with P and T, they change about the same amount for both the reactants and the products. (See the page on the Clapeyron Equation) for more discussion.

    P-T projection with the position of the Grt-in reaction (curved line between the yellow and blue fields) shown in the FeO-MgO-A2O3 ternary animation.
    FeO-MgO-Al2O3 diagram projected from quartz. Note that this is not the normal AFM diagram. Click for animated GIF showing variation in modes with temperature. QuickTime version ( 103kB Apr2 07)
    Many solid-solid reactions are continuous because they involve phases whose compositions are changing as their modes change. This occurs because the phases contain appreciable solid solution. Specific reactions are difficult to express, but schematically, one would be:
    kyanite + orthopyroxene1 = orthopyroxene2 + garnet

    This reaction is shown on the P-T diagram (left) and in the Al2O3-FeO-MgO diagram (right). The P-T diagram is a projection -- garnet and orthopyroxene compositions change along the reaction curve.
  • P-T diagram showing the (dehydration) reaction of brucite to periclase + H2O. Figure from D. Perkins.
    Devolatilization reactions are net-transfer reactions that involve the liberation of a volatile phase (H2O for dehydration reactions or CO2 for decarbonation reactions). Examples of a dehydration and a decarbonation reaction are:
    muscovite + quartz = K-feldspar + sillimanite + H2O, (dehydration)
    calcite + quartz = wollastonite + CO2 (decarbonation)

    Because the entropy of a fluid is generally greater than the entropy of solid phases, fluids appear on the high-temperature side of most such reactions. If the fluid composition is fixed (stays constant), then devolatilization reactions are discontinuous, but if the fluid composition can vary as a result of the liberation of H2O or CO2, then the reactions will be continuous.

    Curvature: The curves for dehydration or decarbonation reactions on a P-T diagram will have shallow slopes at low pressure because the volume of a fluid phase is much larger than that of solid phases. However, the compressibility of a fluid leads to a rapid decrease in volume as pressure rises, so the slopes steepen with rising pressure, leading to substantial curvature. Some (rare) reactions curve around and gain a negative slope at high pressure.

    Sensitivity to fluid composition: The P-T position of a decarbonation or dehydration reaction changes if fluid composition changes. For a dehydration reaction such as:
    muscovite + quartz = K-feldspar + sillimanite + H2O,
    if the rock contains an H2O-rich fluid, the right-hand side of the reaction will be stable over a smaller range of conditions than if the rock contains an H2O-poor fluid (either because the fluid is diluted with CO2, or the rock is ''dried out'' and not fluid saturated).

    Mixed-volatile reactions involve the liberation and/or consumption of more than one volatile phase, typically H2O and CO2. An example is:
    tremolite + calcite + quartz = diopside + H2O + CO2

    Mixed volatile reactions have the same general shape as dehydration and decarbonation reactions on P-T diagrams. (However, they are generally plotted on T-X diagrams instead of P-T diagrams. Click here for more discussion of different types of phase diagrams.)
  • Terminal reactions involve the creation of a new phase from two or three other phases, or (in the other direction) the decomposition of one phase into two or three others. They may be solid-solid reactions or they may involve a fluid phase. An example of a terminal reaction is:
    chloritoid = staurolite + garnet + chlorite
    At lower temperature (583°), chloritoid is stable for compositions that fall within the red, green and yellow 3-phase fields.
    At higher temperature (584°), staurolite, garnet and chlorite are stable together and chloritoid is gone. So, the reaction takes place between 583° and 584° at 1 GPa.
    AFM diagram projected from quartz, muscovite. Click for animated GIF showing loss of chloritoid. QuickTime version ( 67kB Apr2 07)

  • Tie line flip reactions involve two phases becoming stable together that were previously unstable together, and vice versa. As with terminal reactions, they may be solid-solid reactions or may involve a fluid phase. An example of a tie-line flip reaction is:
    chlorite + garnet = biotite + staurolite
    AFM diagram projected from quartz, muscovite. Click for animated GIF showing tie-line flip. QuickTime version (Quicktime Video 261kB Aug8 07)
    Garnet and chlorite are stable together at temperatures below the tie line flip reaction
    Staurolite and biotite are stable together at temperatures above the tie line flip reaction

  • Ion-exchange reactions (generally just called "exchange reactions") involve two phases that both share a solid solution exchange such as NaK or Fe+2Mg such as garnet, (Fe,Mg)3Al2(SiO4)3 and biotite, K(Fe,Mg)3(Si3Al)O10(OH)2 (both formulas are simplified). Reactions of this type are all continuous, and differ from the others kinds of reactions discussed above in that there is no real product or a reactant. Instead, across the complete range of conditions over which the two minerals are stable, both are changing in composition. Curves for these reactions are straight on a P-T diagram (but can only be plotted for specific compositions) and very steep (because there is essentially no change in volume associated with the reaction.

    For Fe+2 and Mg exchange between garnet and biotite, the reaction can be written:
    almandine-in-garnet + phlogopite-in-biotite pyrope-in-garnet + annite-in-biotite, or:
    Fe3Al2(SiO4)3 + KMg3(Si3Al)O10(OH)2Mg3Al2(SiO4)3 + KFe3(Si3Al)O10(OH)2.

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