Olivine (var. Forsterite)

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  1. What is the coordination number of Si? Mg?
  2. What is the relationship of the silica tetrahedra? Are they connected to each other, or are they isolated from each other? (Hint: look at the corners of the Si polyhedra and see what they are connected to in the structure.
  3. What is the relationship of the Si and Mg polyhedra? Are they connected to each other? If so, how are they connected? (i.e., what structural components do they share?)
  4. Orient the structure so that you are looking down the z-axis and the y-axis is to the left. Note the repeating layers of O atoms in the structure along the x-axis direction. What type of close packing is this similar to...HCP or CCP?
  5. Based on your previous answer, can you predict the position of the next series of O atoms if this structure was extended into space (e.g., during crystal growth)?
  6. Compare the olivine structure to the spinel structure. Both have model formulas of XY2O4 (i.e., one tetrahedral site and two octahedral sites). Which mineral appears to have greater symmetry? Which mineral appears to have a greater density?
  7. Olivine and spinel are both found in the Earth's mantle. Based on your answer for the previous question, which structure will be favored at very high pressures, and why?