Unknown #4
Download and view the crystal structure data
- Right click on Unknown_4.amc ( 586bytes Dec30 04) to download the crystal structure to your computer. (This file was modified from an AMC file in the American Mineralogist Crystal Structure Database).
- Once the file has downloaded to your computer, click on it to automatically start XtalDraw and open the file for viewing. You should see something like this...
- Try to resist looking at the mineral chemical formula (Edit > Chemical Formula) as this will probably be too big of a hint right off the bat.
- XtalDraw has colored all the cations the same color. To better see the placement of the different cations in this structure, click on Edit > Atom Attributes to change the colors of the various cation sites.
Determine the identity of the unknown
Using your knowledge of crystallography and systematic mineralogy, and manipulating the downloaded structure in XtalDraw, 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?
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. 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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