Integrating Research and Education > Crystallography > Identifying Minerals Using Chemical and Crystallographic Data > Directions for XtalDraw Users > Unknown #5

Unknown #5

Download and view the crystal structure data

  1. Right click on Unknown_5.amc ( 1kB 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).
  2. 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...
  3. 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.
What is the crystal system of this mineral?
Hexagonal (trigonal sub-system)
Is this mineral a silicate or non-silicate?
showShow question #3
Are the silica tetrahedra isolated or connected in some way? If connected, how?
showShow answer
The silica tetrahedra form rings.
showShow question #3
What is the largest site in this structure? (i.e., what element does it contain?)
showShow answer
showShow question #4
Based on the arrangement of silica tetrahedra, what class of silicate is this mineral?
showShow answer
Cyclosilicate (ring silicate).

showShow question #5
Based on the mineral's composition, do you think this mineral is strongly colored or not?
showShow answer
It contains Fe, which is one of the dominant causes of color in most minerals. In fact, this mineral is so strongly colored, it is jet black. what mineral is this? (extra credit if you can guess the variety name)
Tourmaline (var. schorl) - (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4

Image courtesy of Amethyst Galleries.
The structure is from Foit, F.F. (1989) Crystal chemistry of alkali-deficient schorl and tourmaline structural relationships. American Mineralogist 74:422-431.

Schorl is the variety of tourmaline that has mostly Fe in the (Al,Fe,Mn) site.

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