[MSA-talk] campfire experiments: color change in fluorite

David L. Bish bish at indiana.edu
Sun May 4 09:42:31 EDT 2014

Hi Kent,

We too did similar experiments in the lab with fluorites of a variety of 
colors. When heated to 400C they all lost their color; none fragmented. 
We did a few experiments where we taped chips of these heated samples in 
front of the direct beam on our X-ray diffraction instrument. Previously 
purple fluorites all changed to green after an overnight exposure. This 
behavior appears to be consistent with what "the book" says, that the 
colors are due to color centers.


On 5/3/2014 11:25 AM, Ratajeski, Kent wrote:
> All,
> Having just returned from taking my students to the KY-IL fluorite 
> district, I have something to share with you all. While sitting around 
> the campfire at Cave-in-Rock State Park, my students recalled 
> something they learned in lecture (that's always nice): that some 
> varieties of fluorite owe their color to vacancies within the crystal 
> lattice (color centers), and that by heating these samples and 
> allowing diffusion/recrystallization to occur, these colors can 
> sometimes be removed, producing an uncolored crystal.  My students 
> proceeded to test this by placing different-colored samples (yellow 
> and purple) into tin cans within the center of the campfire.  Not 
> having tried this experiment myself, I predicted that nothing would 
> happen other than fragmenting the samples into pieces.  I was wrong. 
>  Here are the results:
> 1.  All samples of purple fluorite tested fragmented into many 
> sand-sized, somewhat whitish grains.
> 2.  Most yellow samples (apparently the color is from Yttrium) did not 
> fragment at all, but lost all color, turning the crystals totally white.
> Two questions for you all:
> 1.  What is responsible for the purple color (I've heard it is 
> hydrocarbon-rich fluid inclusions), and why would this correlate with 
> increased fragmentation of the crystal?  If it is fluid inclusions, I 
> can certainly understand why they would fragment easily, but if not...??
> 2.  My guess is that micro-fractures formed in the yellow fluorite, 
> and that is what caused the loss of color.  They couldn't have 
> recrystallized at the temperature of the campfire, right?
> Thanks for any insights.  I'd like to report back to the students your 
> expert assessments at the final exam this Wednesday.
> - Kent
> ------------
> Kent Ratajeski, Ph.D.
> 301 Slone Research Building
> Department of Earth and Environmental Sciences
> University of Kentucky
> Lexington, KY 40506-0053
> Phone:  859-257-4444
> Fax: 859-323-1938
> http://www.as.uky.edu/users/krata2
> _______________________________________________
> MSA-talk mailing list
> MSA-talk at minlists.org
> http://lists.minlists.org/mailman/listinfo/msa-talk

David L. Bish
Department of Geological Sciences
Indiana University
1001 E. 10th St.
Bloomington, IN 47405

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.minlists.org/pipermail/msa-talk/attachments/20140504/e0b281a6/attachment.htm>

More information about the MSA-talk mailing list