Unknown #8
Download and view the crystal structure data
- Right click on Unknown_8.amc ( 992bytes 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.
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.
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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).
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The silica tetrahedra are linked in a 3-dimensional network, making this mineral a tectosilicate.
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What crystal system does this mineral belong to?
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Why isn't the symmetry monoclinic?
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Because of ordering of Si and Al tetrahedra in the structure.
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What is the other cation in this mineral besides Si and Al?
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...so what mineral is this?
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Anorthite - CaAl
2Si
2O
8
Anorthite
The structure is from Angel R.J. (1988) American Mineralogist 73:1114-1119.
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.
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