Initial Publication Date: June 28, 2005
Unknown #4
Download and view the crystal structure data
- Right click on Unknown_4.cmdf ( 5kB 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.
What crystal system does this mineral belong to?
Is this mineral a silicate or non-silicate?
Choose the
Transform > Set Range command to expand the crystal structure beyond the unit cell. Clicking on
Expand six times followed by
Apply should be enough.
Then 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 are isolated from one another.
Based on the arrangement of silica tetrahedra, what class of silicate is this mineral?
Nesosilicate (or Orthosilicate)
Consider the composition of this mineral. What group of minerals in the Nesosilicate family does this mineral belong to?
This mineral is one of the aluminosilicate polymorphs.
There appear to be two crystallographically-unique sites for Al in this mineral. Go to the
Atom Info window and change the color for one of the Al sites. Then go to
Edit > Bonding and add the Al-O bonds to the structure. What is/are the coordination(s) of Al in the two sites?
One Al site is octahedral, and the other is tetrahedral.
Consider the two different coordinations of Al in the structure. Do either of these seem relatively high or low compared to the usual coordination of this element in most other minerals? What could this be telling you about the conditions under which this mineral forms (low/med/high P or T)?
The tetrahedrally-coordinated site is a lower coordination than that observed in most other aluminous minerals which have all Al in 6-fold or octahedral coordination. Coordination number tends to decrease with increasing temperature, hence this mineral was probably formed at rather high temperatures (i.e., in high-grade metamorphic rocks).
...so what mineral is this?
Sillimanite - Al
2SiO
5
Sillimanite
Image courtesy of Amethyst Galleries.
The structure is from Winter, J.K., and Ghose, S. (1979) Thermal expansion and high-temperature crystal chemistry of the Al
2SiO
5 polymorphs. American Mineralogist 64:573-586.
Sillimanite is the high-temperature aluminosilicate polymorph. Kyanite (the high-P polymorph) has all Al in 6-fold coordination, while Andalusite (the low-P polymorph) has Al in 6-fold and 5-fold coordination.
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