Chapter 24 Ans

Chapter 24 Ans - Answers for Chapter 24: Stable Mineral...

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Answers for Chapter 24: Stable Mineral Assemblages in Metamorphic Rocks 1. What is meant my mineral paragenesis ? Paragenesis refers to the equilibrium mineral assemblage that characterizes a particular (metamorphic) rock. What minerals are typically excluded in paragenesis ? Relict pre-metamorphic minerals or later alteration products are excluded. How does paragenesis differ from the similar sounding petrogenesis ? Petrogenesis refers to the processes by which a rock was created, not the mineral assemblage that develops. 2. Goldschmidt’s mineralogical phase rule is φ = C . Why should this be true for a typical metamorphic rock? φ generally equals C because a typical metamorphic rock occurs in a divariant situation (between isograds and not coincidentally on one). If F = 2, then φ = C – 2 + F = C – 2 + 2 = C 3. What are the implications for a metamorphic rock in which φ < C ? Solid solution is typically involved, so that two or more components may be contained in one or more phases. 4. One alternative explanation for a metamorphic rock in which φ > C is that equilibrium was not attained. Explain why disequilibrium would tend to raise φ . The phase rule is only applicable to systems in equilibrium so that components are exchanged among the phases in a systematic way. Any number of phases can coexist (or even the same phases with different compositions) if equilibrium is not attained. 5. Another alternative explanation for a metamorphic rock in which φ > C is that the sample was collected exactly on an isograd. Explain how this might be possible. If a sample is collected on an isograd, both the reactants and products of the isograd reaction will be present. That means more phases than would occur between isograds where only the stable products are present. 1
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6. A “perfectly mobile” component is one for which the chemical activity is controlled externally to the system, whereas the activity of other components is controlled within the system. As a slight simplification, let’s treat activity as though it were composition . Consider a system of your own creation in which a saline NaCl solution acts as a pore fluid able to flow through a porous container of sand-sized grains of quartz and halite. Devise a simple system in which Na + and Cl - would be considered perfectly mobile (concentrations controlled externally) and another in which they would not (concentrations controlled internally). Describe the two systems (using the mineralogical phase rule), both before the fluid is added and after equilibrium is attained. In a perfectly mobile situation there is an excess of NaCl-undersaturated water flowing through
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Chapter 24 Ans - Answers for Chapter 24: Stable Mineral...

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