metamorphic_reactions - 1 Deriving and Balancing...

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1 Deriving and Balancing Metamorphic Reactions Dexter Perkins (University of North Dakota) Given a list of minerals, how do we determine all the possible reactions that can occur between them? There are many ways to approach this question and the answer can sometimes be quite elusive. One Way To Proceed •Make a list of all the phases and their formulas. •Identify the chemical system and the number of components •Use the phase rule to determine how many phases are in a normal univariant reaction •Make a list of all possible reactions, identifying them by the phases they DO NOT contain •Balance the reactions Example Involving 5 Phases and 3 Components Consider the phases: •Wo: Wollastonite CaSiO 3 •Ky: Kyanite Al 2 SiO 5 •An: Anorthite CaAl 2 Si 2 O 8 •Gr: Grossular Ca 3 Al 2 Si 3 O 12 •Qz: Quartz SiO 2 The chemical system is CaO-Al 2 O 3 -SiO 2 ; it contains 3 components. The phase rule tells us that a univariant reaction will include 4 phases (unless it is degenerate). So, each reaction will be missing one phase. Possible reactions are (absent phase in parentheses): 1. (Wo) involves Ky, An, Gr, Qz 2. (Ky) involves Wo, An, Gr, Qz 3. (An) involves Wo, Ky, Gr, Qz 4. (Gr) involves Wo, Ky, An, Qz 5. (Qz) involves Wo, Ky, An, Gr Consider the first reaction (Wo). It involves kyanite, anorthite, grossular and quartz. Inspection Method: For fairly simple systems involving few phases, we can usually identify and balance reactions by inspection. For this example, note that only two of the phases (An, Gr) contain Ca, so they must be on opposite sides of the reaction. Also, Gr contains 3 Ca, compared to 1 in An. Therefore we can start with:
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2 something + 3 An = something + Gr something + 3 CaAl 2 Si 2 O 8 = something + Ca 3 Al 2 Si 3 O 12 If we next consider balancing Al, it becomes apparent that we must put 2 Ky on the right hand side: something + 3 An
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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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metamorphic_reactions - 1 Deriving and Balancing...

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