Spinel

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Download the spinel unit cell ( 11kB Jun8 07). This is a cmdf file which you can view with CrystalMaker.

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Open the Crystal Structures Library on the CrystalMaker disc, and click on Minerals > Non-Silicates > Oxides > Spinel Group > Spinel Series (All) > Spinel.

For viewing with XtalDraw (more info) ,

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Click on the SPINEL file in the XtalDraw folder.

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Go to Crystal Structures (more info) and choose "Spinel lattice", which is listed under "Some common cubic lattices". This resource is currently off-line. Please check back at a later date.

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Questions

1. What is the coordination number for Mg? for Al?
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CN of Mg = 4; CN of Al = 6
2. What is the strength of the bonds around each Mg cation? What is the strength around each Al cation? (To do these calculations, divide cationic charge by the number of bonds.)
3. Consider a single oxygen anion. What cation(s) is it bonded to? Add up the strength of the bonds (you just calculated them). Does the total add up to oxygens anionic charge? Why or why not?
4. Note the overall cubic form of this structure. Orient the structure so you are looking down the body diagonal (the projection of the three crystallographic axes on your screen should be pointing away from each other at 120° angles). The oxgyen atoms "nearest" you form the top layer of a close-packed array; successive layers are "away" from you within the structure. What type of close packing is this: HCP or CCP?
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If your visualization program allows you to remove individual atoms from the structure (e.g., the "zapper" tool in CrystalMaker), remove an oxygen atom to determine what is immediately below it in the structure.
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CCP
5. Notice that the body diagonals of the structure are 3-fold axes. How many 3-fold axes are present?
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Four.