Unknown #8
Download and view the crystal structure data
- Right click on Unknown_8.cmdf ( 12kB Dec16 09) 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.
-
Click on
Edit > Bonding to draw the Si-O bonds. Click on
Add and choose "Si" under
From and "O" under
To. Follow the same procedure for adding the Al-O bonds. 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 tetrahedra, use the
Model > Polyhedral command to plot the structure as a polyhedral model).
-
The silica tetrahedra are linked in a 3-dimensional network, making this mineral a tectosilicate.
-
What crystal system does this mineral belong to?
-
-
Why isn't the symmetry monoclinic?
-
Because of ordering of Si and Al tetrahedra in the structure.
-
What is the other cation in this mineral besides Si and Al?
-
...so what mineral is this?
-
Anorthite - CaAl
2Si
2O
8
Anorthite
The structure is from Wainwright & Starkey (1971) Z Kristallogr 133:75-84.
The amount of Al in the tetrahedral site in this mineral is enough to make it a feldspar, but not enough to make it a feldspathoid. The presence of Ca as the sole non-tetrahedral cation indicates that it is Anorthite.
Show question #1
Hide
