Unknown #5
Download and view the crystal structure data
- Right click on Unknown_5.cif ( 16kB Dec14 04) to download the crystal structure to your computer. (This file was modified from a CIF file in the Crystal Structure Library provided in CrystalMaker).
- Start the CrystalMaker program.
- Choose the File > Open command, and select the file you just downloaded to view in CrystalMaker.
- Click on OK to generate the crystal structure for viewing. You should see something like this...
Determine the identity of the unknown
Using your knowledge of crystallography and systematic mineralogy, and manipulating the downloaded structure in CrystalMaker, answer the following questions to deduce the identity of the unknown. Check your answers by clicking on the "show answer" tabs which appear with each question.-
What is the crystal system of this mineral?Hexagonal (trigonal sub-system)
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Is this mineral a silicate or non-silicate?Silicate
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Click on Edit > Bonding to draw the Si-O bonds. Click on Add and choose "Si" under From and "O" under To. Clicking on the box under Info will give you information about coordination. Finally, click on OK to draw the bonds. The window should look like this...
Are the silica tetrahedra isolated or connected in some way? If connected, how? (hint: to highlight the silica tetrahedra, use the Model > Polyhedral command to plot the structure as a polyhedral model).The silica tetrahedra form rings.
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Keeping the Edit > Bonding window open, add the following bonds to the structure: Fe-O, Al-O, B-O, Na-O, and H-O. Then examine at the polyhedral model for the entire structure.
What is the largest site in this structure? (i.e., what element does it contain?)Sodium -
Based on the arrangement of silica tetrahedra, what class of silicate is this mineral?Cyclosilicate (ring silicate).
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Based on the mineral's composition, do you think this mineral is strongly colored or not?It contains Fe, which is one of the dominant causes of color in most minerals. In fact, this mineral is so strongly colored, it is jet black.
Show question #1
Hide...so what mineral is this? (extra credit if you can guess the variety name)
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Tourmaline (var. schorl) - (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4The structure is from Zhang L, Ahsbahs H, Hafner SS, Kutoglu A (1997) Single-crystal compression and crystal structure of clinopyroxene up to 10GPa. American Mineralogist 82:245-258.
Schorl
Image courtesy of Amethyst Galleries.
Schorl is the variety of tourmaline that has mostly Fe in the (Al,Fe,Mn) site.
