Name __________________ Petrology — Spring 2006 Igneous rocks lab — Part III Phase equilibria — Two Component Systems Due Tuesday 3/7 The system below is an isobaric system (fixed pressure). The Phase Rule can be modified for restricted variance by subtracting 1 F for each such restriction: F = C – φ + 1 (isobaric — also applies to isothermal, etc.) f = the number of phases: the number of different types of mechanically separable portions of the system C = the minimum number of chemical constituents that are required to define the composition of every phase in the system. F = the variance or number of degrees of freedom — the number of intensive parameters that must be specified in order to completely determine the state of a system. Alternatively, F is the number of intensive parameters that can be changed independently without changing the parameters of the phase rule (i.e., losing or gaining a phase). Consider the following system:Name __________________ Petrology — Spring 2006 1: Suggest a natural isobaric system in which a silicate liquid might crystallize. 2: Suppose you mix 4 grams of pure fayalite with 16 grams of pure forsterite. Express the formula as Foxx (weight fraction, equal to grams of forsterite/(grams of forsterite + grams of fayalite). 3: Mark a point and label it “a” for the appropriate bulk composition of question #2 at 1950°C. Determine the variance (number of degrees of freedom) of the system at equilibrium under these conditions. 4: As composition “a” cools, at what temperature will φ change? What occurs at this temperature? Describe the composition of each phase present. Label this point “b”. 5: Determine the variance at this point. Are there more or fewer possible variables than at 1950°C? 6: Describe what occurs as the system cools a few degrees from this point. What happens to the amount and composition of each phase? 7: Does this represent a continuous or discontinuous reaction? Why?Name __________________ Petrology — Spring 2006 8: Determine the ratio of olivine to liquid at 1700°C. Clearly label and show your work. 9: At what point will equilibrium crystallization be complete? Why? What is the composition of each phase when the last drop of liquid disappears? 10: Fractional crystallization is the removal of crystals when they are formed so that they can no longer react with the liquid. Suggest a method by which this can be accomplished in nature. 11: How would fractional crystallization affect the path of crystallization in the example above? What would be the affect on the composition of the final liquid and solid? Explain your answer by summarizing the effect of fractional crystallization on composition in a general statement that would apply to both Fo-Fa and Ab-An. 12: What is the temperature and composition of the first melt produced when Fo50 is melted? 13: What is the temperature and composition of the last solid to melt (assuming equilibrium melting of Fo50)? 14: Partial melting involves the removal of increments of melt as they are produced. Why might this be practically an inevitable process in nature? 15: Describe in detail the partial melting of solid Fo30 until it is completely molten. Draw the melting path of all phases involved on the phase
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