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Open the Crystal Structures Library on the CrystalMaker disc, and click on Minerals > Silicates > Tectosilicates > Feldspars > Sanidine.
Click on the SANIDINE file in the XtalDraw folder.
- What is the coordination numbers of Si, Al, and K in the sanidine structure?
- What ions are in tetrahedral coordination in this structure? (Look carefully!) How are the tetrahedra connected to each other
- Look down the b-axis and rotate the model around this axis. Do the atomic positions exactly repeat themselves after 90°, 120°, or 180° rotations? (Hint: look down the center of the "channels" as seen in this orientation.)
- All of the tetrahedral sites are essentially the same in this model. What would happen to the symmetry if the structure was "ordered" such that every fourth tetrahedron was somehow unique (e.g., in its size or shape): would it increase or decrease?
The symmetry would decrease. In fact, such an ordering occurs among the K-feldspar polymorphs microcline and orthoclase, whereby Al substitutes for Si in particular tetrahedral sites. The structure of the high-T polymorph sanidine is said to be "totally disordered", and no specific tetrahedral site is always occupied by Al.