reaction_types.v2

reaction_types.v2 - 1 Reaction Types Curves Handout Dexter...

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1 Dexter Perkins, Dept. of Geology, University of North Dakota. . (Based heavily on material provided by Dave Hirsch, Western Washington University) 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 reactions on Figure 1.) On either side of the curve, a different phase, or 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 . For all discontinuous reactions, the phase (or combination of phases) with the lowest volume is stable on the high pressure side of the reaction curve. The phase (or combination of phases) with the greatest entropy is stable on the high temperature side of the curve. Because volume and entropy are roughly proportional for most minerals, this means that most discontinuous reactions have positive slopes. There are, Figure 1 . Phase diagram for the Al 2 SiO 5 polymorphs.
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2 however, exceptions, especially for reactions that involve a fluid phase (H 2 O or CO 2 ). # Continuous reactions
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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reaction_types.v2 - 1 Reaction Types Curves Handout Dexter...

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