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

Alfredo Petrov alfredo at mindat.org
Mon May 5 00:31:09 EDT 2014

When heating crystals in tin cans in a campfire, try what primitive gem
miners do to heat treat their gemstones: Pack sand around the crystals, ie.
fill the can with sand. This slows down the heating and reduces thermic
shock, resulting in less fracturing.

On 4 May 2014 22:42, David L. Bish <bish at indiana.edu> wrote:

>  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.
> Cheers,
> Dave
>  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
> _______________________________________________
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> --
> David L. Bish
> Department of Geological Sciences
> Indiana University
> 1001 E. 10th St.
> Bloomington, IN 47405812-855-2039
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