Unknown #10
Download and view the crystal structure data
- Right click on Unknown_10.cif ( 2kB Dec13 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.
Show question #1
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What crystal system does this mineral belong to?
Hexagonal
Is this mineral a silicate or non-silicate?
Non-silicate
Click on Edit > Bonding to draw the C-O bonds. Click on Add and choose "C" 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...
What element is C bonded to in this mineral, and what is the coordination of C? (hint: use the Model > Polyhedral command to plot the structure as a polyhedral model).
What element is C bonded to in this mineral, and what is the coordination of C? (hint: use the Model > Polyhedral command to plot the structure as a polyhedral model).
Each carbon is bonded to three oxygens, forming triangular CO3 groups.
What class of non-silicate minerals does this belong to?
Carbonate
Go back to Edit > Bonding and add the Ca-O bonds to the structure. What is the coordination of Ca in this mineral?
Octahedral
...so what mineral is this?
Calcite - CaCO3
Calcite
Image from the New York State Academy of Mineralogy
The structure is from Graf, DL (1961) Crystallographic tables for the rhombohedral carbonates. American Mineralogist 46:1283-1316
The only other polymorph of CaCO3 is aragonite, but it is orthorhombic.
CalciteImage from the New York State Academy of Mineralogy
The structure is from Graf, DL (1961) Crystallographic tables for the rhombohedral carbonates. American Mineralogist 46:1283-1316
The only other polymorph of CaCO3 is aragonite, but it is orthorhombic.
