Unknown #3
Download and view the crystal structure data
- Right click on Unknown_3.amc ( 359bytes Dec29 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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What class of non-silicate minerals does this mineral belong to?
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What crystal system does this mineral belong to?
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What cleavage form would be exhibited by this mineral?
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Is the coordination of all Fe's in this mineral the same, or is there more than one crystallographic site that accepts Fe?
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There are two Fe sites: one tetrahedral and the other octahedral.
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What do the two sites for Fe imply about the oxidation state of Fe in this mineral?
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Ferrous (Fe2+) and ferric (Fe3+) iron are present.
...so what mineral is this?
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Magnetite - Fe
3O
4
Magnetite
Image courtesy of Amethyst Galleries.
The structure is from Finger L W, Hazen R M, (1986) Physics and Chemistry of Minerals 13:215-220.
You should have realized that the mineral was not hematite because of the isometric crystal structure (hematite is hexagonal) and because of the multiple valences of Fe (hematite only has Fe3+).
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