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Rare Green Fluorescent Fluorite, Fourmile Creek, Colorado

1.11.2017

Fluorite is a mineral that occurs in many different colors and habits, and is often fluorescent under UV.  Most commonly, fluorite shows a deep, intense blue fluorescence under long wave UV, but some varieties are known to respond with different colors under UV.  Green is not a common color of fluorescence for fluorite, but an unusual purple-grey colored botryoidal form of this mineral, found in the Fourmile Creek area, near Canon City, Fremont Co., Colorado, shows a rare green response under short wave UV.  Here is an example of such a specimen.  The size of this piece is approximately 10 x 8 x 6 cm, and it weighs 376 grams.

Green short wave UV fluorescent response of botryoidal fluorite from the Fourmile Creek area of Colorado.

 

This specimen also shows a dimmer, blue fluorescent response under long wave UV.

Dim blue short wave UV fluorescent response of the same specimen.

 

In addition to its unusual fluorescent response under UV, the fluorite from Fourmile Creek exhibits an uncommon botryoidal habit and purple-grey color when viewed under visible light.  The fluorite has formed on the surface of an unidentified red/brown non-fluorescent matrix.

Visible light image showing the unusual botryoidal habit and grey-purple color of this fluorite specimen from Fourmile Creek, Colorado.

 

As seen in the following close-up images of a broken surface showing a cross section of one of the hemispherical regions (seen at lower left in the above series of photographs), the fluorescence is not merely a surface phenomemon, but occurs throughout the interior of the fluorite.

Short wave UV (left) and visible light (right) images showing a close-up view of the interior of one of the botryoidal masses of fluorite.  The concentric banded pattern of fluorescence suggests that the fluorescent activator was present in varying concentrations in the hydrothermal fluid during the entire depositional process of fluorite formation.

 

This specimen has an additional exposure of botryoidal fluorite on a separate face.

 Mouse over the scrolling images for UV wavelength information.

 

In his book, Fluorescence- Gems and Minerals Under Ultraviolet Light (1994), Manuel Robbins speculates that the mechanism of green fluorescence in these Fourmile Creek fluorites is likely due to uranyl-activation.  In fact, this specimen is radioactive, having a measured emission of approximately 200 cpm.  This finding is indicative of the presence of uranium in this specimen, suggesting that Robbin's hypothesis for uranyl-activation is correct.

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