Initial Publication Date: July 14, 2005

Forsterite

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Questions

  1. Look at a single Mg cation. How many oxygens does it bong to?
  2. Look at a single Si cation. How many oxygens does it bong to?
  3. Calculate the bond strengths around Mg and Si.
  4. Now look at a single O anion. What does it bond to? How many bonds are around it and what are their strengths (you just calculated them). Add up the bond strengths—does it match oxygen's anionic charge? Why or why not?
  5. Each Si4+ is surrounded by four O2- making a tetrahedron. What is the relationship between the silicon tetrahedra. Do they share O2- to make sheets or chains or networks, or are the related in some other way?
  6. 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)?
  7. 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 has greater symmetry? Which has the greater density?
  8. 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?