Forsterite

Download the structure for interactive viewing

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For viewing with the demo version of CrystalMaker (more info)

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Go to Silicate Minerals at Stephen Heyes' website at Oxford University and click on the image of the forsterite crystal structure to download and open in CrystalMaker for viewing.

For viewing with the commercial version of CrystalMaker (more info)

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Open the Crystal Structures Library on the CrystalMaker disc, and click on Minerals > Silicates > Orthosilicates > Olivine Group > Olivine-Forsterite.

For viewing with XtalDraw (more info)

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Click on the OLIVINE file in the XtalDraw folder.

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Questions

1. Look at a single Mg cation. How many oxygens does it bong to?
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Each Mg is bonded to 6 oxygens.
2. Look at a single Si cation. How many oxygens does it bong to?
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Each Si is bonded to 4 oxygens.
3. Calculate the bond strengths around Mg and Si.
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BS for Mg = 2/6 = 1/3; BS for Si = 4/4 = 1.
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 O^2- making a tetrahedron. What is the relationship between the silicon tetrahedra. Do they share O^2- to make sheets or chains or networks, or are the related in some other way?
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The Si tetrahedra are not connected in the forsterite structure.
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)?
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If you are using CrystalMaker, you can perform this operation by increasing the plot range of the model by clicking on Transform -> Set Range and increasing the maximum range for the x-axis to 1.8 angstroms.
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?
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Spinel has the greater symmetry and 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?
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The spinel structure will be favored because of its greater density.