Initial Publication Date: June 28, 2005
Unknown #1
Download and view the crystal structure data
- Right click on Unknown_1.cmdf ( 4kB 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?
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)
Go back to
Edit > Bonding and add the Al-O bonds to the structure. What is the coordination of Al in this mineral?
Now add the Mg-O bonds to the structure. What is the coordination of Mg in this mineral? Is this the "normal" coordination for Mg exhibited by most other rock-forming minerals? What might this be telling you about the conditions under which this mineral forms (low/med/high P or T)?
Mg is in 8-fold (dodecahedral) coordination. In most other minerals, Mg is in 6-fold (octahedral) coordination. Coordination number tends to increase with increasing pressure, hence this mineral was probably formed at rather high pressures (i.e., in metamorphic rocks).
...so what mineral is this?
Pyrope Garnet - Mg
3Al
2Si
3O
12
Pyrope
The structure is from Hazen, RM and Finger, LW (1989) High-pressure crystal chemistry of andradite and pyrope: Revised procedures for high-pressure diffraction experiments. American Mineralogist 74:352-359
Pyrope is the Mg-endmember in the "Pyralspite" group of garnets.
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