Initial Publication Date: June 28, 2005
Unknown #2
Download and view the crystal structure data
- Right click on Unknown_2.amc ( 935bytes 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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Is this mineral a silicate or non-silicate?
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Are the silica tetrahedra isolated or connected in some way? If connected, how?
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The silica tetrahedra form chains.
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Based on the arrangement of silica tetrahedra, what class of silicate is this mineral?
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Inosilicate (chain silicate).
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Based on the repeat unit along the chains, is the mineral a pyroxene or a pyroxenoid? (Hint: Use
View > Alter Structure Limits to modify the dimensions of the structure to see longer pieces of the chains of silica tetrahedra)
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Since the silica tetrahedra repeat on an alternate basis (every other one), the mineral is a pyroxene
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What is the crystal system of this mineral? Based on your answer, is this a clinopyroxene or an orthopyroxene?
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This mineral is monoclinic, so its a clinopyroxene.
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Based on the location of cations in the structure, is this an end-member of the pyroxene quadrilateral?
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...so what mineral is this?
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Hedenbergite - CaFeSi
2O
6
Hedenbergite
The structure is from Zhang L, Ahsbahs H, Hafner SS, Kutoglu A (1997) Single-crystal compression and crystal structure of clinopyroxene up to 10GPa. American Mineralogist 82:245-258.
Hedenbergite is the Ca-Fe endmember clinopyroxene.
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