Diopside

Download the structure for interactive viewing

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

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Open the Crystal Structures Library on the CrystalMaker disc, and click on Minerals > Silicates > Chain Silicates > Pyroxenes > Diopside.

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Click on the DIOPSIDE file in the XtalDraw folder.

Questions

1. What is the coordination of Si, Mg, and Ca in the diopside structure?
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CN of Si = 4; CN of Mg = 6; CN of Ca = 8.
2. Look at the silica teterahedra. How are they connected to each other? That is, do they share 0, 1, 2, 3, or all 4 oxygens?
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The silica tetrahedra are connected by their corners with "bridging oxgyens".
3. Orient the structure so you are looking down the axis of the silicon tetrahedral chain (down the c-axis.) Note that for each of the tetrahedra, three of the oxygens are roughly in the same plane, and the the fourth oxgyen in each tetrahedron "points" either up or down. What is this oxygen connected to?
4. Are any of the chains of Si tetrahedra directly connected to any other chain?
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No.
5. What holds the chains together?
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Each chain of tetrahedra is connected to an adjacent octahedral layer (containing Mg) by the fourth oxygen (noted above), and this octahedral strip will also be connected to a second tetrahedral chain on its other side. This produces the T-O-T structure of alternating tetrahedral-octahedral-tetrahedral layers, commonly referred to as the "I-beam structure".
6. With the structure in this orientation, compare with the directions of the cleavage planes illustrated in your mineralogy textbook and see if you can find their location in the structure. What are the relative bond strengths within the Si tetrahedra compared with bonds involving octahedral sites?
7. Rotate the structure so that the c-axis is horizontal left to right and you are looking directly down the b-axis. Look only at the Ca atoms on the front most layer of the structure. If you connect the centers of four nearest Ca atoms, you will get an outline of the unit cell. The angle at the corner of the unit cell is beta—the angle between the a and c axes. About what angle do you see?