Posted On Mindat.org January 13, 2014 by Frank Roberts
Last week, while opening a second pegmatite exposure at my Emerald Ridge topaz site near Mason, Texas, Erin Balzrette and I began to uncover large gemmy fluorite crystals of a color and clarity not seen before from the site. For nearly a year now, fluorite masses and crystals ranging from 1 to 25 cm have been found in the gem pockets and in the soil around the primary pegmatite exposure. Almost all of the fluorite found to date has been in various shades of green, but crystalline pieces of purple, clear, yellow, amber, brown, white, light blue and even blood-red fluorite have been found.
We were immediately impressed by the new material from the second pegmatite because of its gemmy luster and the intense saturated green color it displayed in the sunlight.
Much of this new material is facet grade. Nearly complete octahedra up to 9 cm in girth have been recovered. Almost immediately, we began finding nodules of fluorite approximately the size and shape of biscuits, having a rich teal color, much of it displaying phantoms and wisps of dark blue in transmitted sunlight. Many of these deeply colored "biscuits" have a drusy outer coating of pinkish-orange microcline and sugary quartz crystals, evidence of their hydrothermal pegmatite origin. The outer coating gives these pieces the appearance of having been breaded and deep fried, yet light readily passes through them, revealing their true internal colors. When we began cleaning them, we noticed something amazing.
After cleaning, the pieces that we collected for their unusual bluish-green color, were no longer bluish-green! Instead we began to find a lot of beautiful reddish-violet specimens that had seemingly replaced identically sized teal specimens. The mystery was soon solved when, after flash photography, the images were again bluish-green. We had collected fluorite that displayed different coloration, depending on the color temperature of the light under which they were observed. I have posted two photos of a typical "biscuit" backlit with both daylight fluorescent and incandescent lighting. The color change is striking.
Evidently there are multiple types of color centers present in the crystal lattice to cause this effect. Evidence for this comes from this fluorite's thermoluminescence. Instead of the usual cyan luminescence typical of fluorite containing "F" type color centers, this material first emits yellow light that slowly migrates through peach, to the red end of the visible spectrum. It quickly takes on a burnt orange luminescence that gradually shifts to a dark red color, reminiscent of glowing coals. This fluorite is bizarre stuff!
I have seen faceted color change fluorite offered for sale by questionable dealers, but never thought I'd see it in the rough. I guess it does exist. The next step will be to have it analyzed to try to determine if the color change effect is due to multiple color center lattice defects, or if it's due to one or more rare-earth chromophores present as trace impurities in the lattice.
More to come.
Teal To Hot Pink Color Change
All above photos from Emerald Ridge, Mason, Texas