Olivine (var. Forsterite)

Download the structure for interactive viewing

Choose one of the options below, based on which program you want to view the structure with.

For viewing with XtalDraw (more info) ,

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Click on the OLIVINE file in the XtalDraw folder.

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.

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Questions

1. What is the coordination number of Si? Mg?
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CN of Si = 4; CN of Mg = 6
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.
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The silica tetrahedra are isolated from each other.
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?)
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Answer
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?
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HCP
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)?
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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.
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?
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Spinel has the greater symmetry and the 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?
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The spinel structure will be favored because of its greater density.