Second Midterm, Fall 2000 (100 pts)

1. (20 pts) Sketch a diagram showing the structure of the earth. Make a table and define the boundaries of each region by
        a. depth
        b. temperature and
        c. pressure
    Be sure to include the surface and the center of the earth in your answer.
2. (20 pts) For each illustration:

a) Identify the structures that are illustrated.
b) Provide representative chemical formula for each.
c) From which region of the earth is this mineral most likely to occur.

3. (20 pts)
a. Draw the ternary diagram of the feldspars.
b. Describe and explain their mixing properties.
4. (20 pts) Below is a PT plot that shows the phase boundaries of the common silica polymorphs. Identify the stable phase associated with each region.
5. (10 pts) Examine the PT diagram that shows the phase boundaries between olivine-b phase-g spinel-perovskite + magnesio-wüstite for pure Mg2SiO4.
a. Using the positions of the lines that indicate the 410, 520 and 660 km discontinuities, determine the values of P and T at these discontinuities.
b. Draw the geotherm onto the figure.
c. Label the regions of the lower and upper mantles and the transition zone.
6. (10 pts) The figure shows the shear wave velocities versus depth (i.e. pressure) determined in the laboratory for olivine, b-phase and g-phase as bold lines. The light line (labeled PREM) is the measured shear velocity profile determined by experimental seismology. The change in shear wave velocity at the 410 km discontinuity is observed to be about 5%. The measured change in shear wave velocity between olivine and b-phase is about 12%.

Estimate the percentage of olivine in the mantle from this diagram.