If you combine 5 grams of NaAlSi3O8 with 3 grams of CaAl2Si2O8 what is the weight fraction of albite in the resulting plagioclase? Show your work. What is the weight fraction of anorthite? Express the plagioclase composition as Anxx (by weight). 8. Plagioclase contains Na, Ca, Al, Si, and O. For phase rule treatment, what is C? Why? It would require 3-dimensions to illustrate the P-T-X variables for a 2-component system, so we simplify our diagrams by concentrating on the most important variables, typically T and X. We thus construct T-X diagrams at some specified pressure. 9. The temperature-composition (T-X) Ab-An binary phase diagram at 1 atmosphere pressure is shown on the right. If we were to heat pure anorthite at this pressure, what would be the variance (F) when it began to melt? Show your work. What intensive variables specifically addressed by the diagram could vary between the first occurrence of melt and the last bit of anorthite to disappear? What extensive variables on the diagram most obviously change during that interval? with composition a at 1550oC. What is the weight fraction of An in the liquid () ?
47. Why must the liquid composition reach point x with further cooling? 48. What mineral forms when the liquid reaches x and what is the variance? What is the reaction and is it continuous or discontinuous? Why? 49. Will the liquid composition reach point y? Why or why not? 50. A liquid of Xbulk = d has an interesting variation on the evolution path described above. As with Xbulk = c, forsterite forms first, followed by enstatite as the liquid composition reaches and evolves along the peritectic curve. Note, as deduced previously, that the Xbulk always lies within the triangle with corners equal to the three coexisting phases. In other words, the whole must lie within the geometry of the parts. But at some point along the peritectic En-Xbulk-Xliq become collinear so that the Xbulk lies on the edge of the 3-phase triangle, not within it. How much forsterite can remain from the reaction you proposed in question 46? Explain. 51. What is the new assemblage and variance after that? 52. On the basis of your answers to Question 50, what is the new continuous reaction and liquid evolution path immediately after Fo disappears? Explain. 53. Outline the remainder of the liquid evolution until the system is completely solid. At what point do new minerals form, and what are they? What is the variance at each stage, and what reactions occur. Are the reactions continuous or discontinuous?
Convert the Fe2O3 and FeO in the basalt in Table 8.3 to FeO* Explain in a single sentence what a norm attempts to do. How does a norm differ from a mode? How would you recognize a silica-oversaturated plutonic rock from a silica-undersaturated plutonic rock? Suppose you have the chemical analysis of a volcanic glass. How would you determine whether it represents a silica-oversaturated or undersaturated composition? Suppose you have chemical analyses for two volcanic with identical wt. % SiO2, but one has a higher alkali/Al2O3 ratio. Which is more likely to be silica-undersaturated? Why? Given the following mineral compositions, explain the trends in Fig. 8.2 assuming that fractional crystallization is responsible. Olivine: (Mg,Fe)2SiO4 Orthopyroxene: (Mg,Fe)SiO3 Clinopyroxene: Ca(Mg,Fe)Si2O6 Albite: NaAlSi3O8 Anorthite: CaAl2Si2O8 Orthoclase: KAlSi3O8 Quartz: SiO2
Why is this a rather difficult distinction to make? In what ways may changing temperature affect metamorphic mineral assemblages and textures? What is a metamorphic field gradient, and how do they differ from a geothermal gradient (at the time of metamorphism)? Describe a situation in which a metamorphic field gradient and a geothermal gradient would be the same. What is lithostatic pressure? What would be the shape of a strain ellipsoid for rocks experiencing lithostatic pressure? How does flattening differ from shear?
Why garnet occur in some medium-grade metabasalts but not in others in the same field area as in Review Question 9? Sketch an x-y-z triangular chemographic diagram with x at the top, y on the left and z on the right (as in Figure 24-2). Suppose you wanted to project a mineral with the formula xy2z2 from the mineral x3y to the y-z pseudo-binary. What would be your formulas for calculating y' and z', and where would xy2z2 plot? Compare the plot point mathematically and graphically. Problems 1. Given the following mineral compositions (Fe is Fe+2 unless indicated): Staurolite (St) (Fe,Mg)2Al9O6[(Si3Al)O16](OH)2 Chloritoid (Ctd) (Fe,Mg)2Al4O2[Si2O8](OH)4 Anorthite (An) CaAl2Si2O8 Glaucophane (Gln) Na2Mg3Al2[Si8O22](OH)2 Vesuvianite (Ves) Ca10(Fe,Mg)2Al4[Si2O7]2[SiO4]5(OH)4
At a higher yet, minerals a and x are no longer stable together. Show/explain how you can use the chemographics to determine the approximate (unbalanced) reaction involved with this isograd #2 (assuming that mineral y does not become stable until higher grades). Balance the reaction. Why might isograd #2 make a poor field-based (first appearance of a mineral) isograd? Why might some rocks show no evidence of the reaction? Be specific and refer to your diagram. What would you name isograd #2 and the zone above it? Why?