Unknown #2
Download and view the crystal structure data
- Right click on Unknown_2.cif ( 2kB 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.-
Is this mineral a silicate or non-silicate?Silicate
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To see an adequate portion of the structure (not just the unit cell), choose Transform > Set Range, and click on Expand 7 times to extend the structure along all axes by 70%. Click OK.
Next, 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 chains.
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Based on the arrangement of silica tetrahedra, what class of silicate is this mineral?Inosilicate (chain silicate).
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Based on the repeat unit along the chains, is the mineral a pyroxene or a pyroxenoid?Since the silica tetrahedra repeat on an alternate basis (every other one), the mineral is a pyroxene
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What is the crystal system of this mineral? Based on your answer, is this a clinopyroxene or an orthopyroxene?This mineral is monoclinic, so its a clinopyroxene.
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Based on the location of cations in the structure, is this an end-member of the pyroxene quadrilateral?Yes.
Show question #1
Hide...so what mineral is this?
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Hedenbergite - CaFeSi2O6
Hedenbergite
The 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.
Hedenbergite is the Ca-Fe endmember clinopyroxene.
