Articles

By Peter Rosewarne

Introduction

The branch of mineralogy dealing with fluorescence apparently gained popularity in the 1930s with the availability of battery-powered portable ultraviolet (UV) lamps. The pioneer in producing such UV lamps and using them to prospect for and showcase minerals was Thomas S Warren, after whom the Thomas S Warren Museum of Fluorescence at Sterling Hill Mine Museum in the USA is named.

Those of you who have been paying attention to previous MinChat articles will remember or knew or guessed already that fluorescence is named after the common mineral fluorite, the earliest mineral to have been observed showing this phenomenon. This may even date back to antiquity with Pliny the Elder having remarked upon “chrysolampis”, a stone, “pale by day but of a fiery lustre by night”, which could have been fluorite.

The causes of fluorescence are certain disruptive factors or activators in the crystal lattice. Only about 10 per cent of minerals exhibit fluorescence. Fluorescence is grouped under the umbrella-term luminescence which is a collective term for the ways in which a substance emits visible light under the influence of certain rays. 

Background

And now, as usual, some mildly technical stuff. There are over 500 minerals that exhibit fluorescence, the more common ones including fluorite, calcite, willemite, scheelite, apatite, aragonite, wollastonite and sphalerite. Ultraviolet light comes in short and long wavelengths with more minerals reacting to short-wave than long-wave, while some react to both. Some always glow with the same colour while others are site-specific, e.g. calcite from Franklin is always orange/red. Minerals usually cease to react when the UV source is turned off, while some continue to react for seconds or longer, a property known as phosphorescence.

Some minerals fluoresce even in direct sunlight and do not need a UV light to show this characteristic. For example, fluorite from the Rogerley Mine in Durham, UK, shows striking fluorescence in sunlight, changing from a deep green to a deep blue, as shown in Figures 1 and 1b, respectively. In the fluorite article of January-February 2021 this effect was mistakenly called dichroism.

Figure 1


Figure 1b

Fluorescence can be a useful way of determining if a mineral specimen is a fake, as glue will show up with a different response to the mineral. It can also be used to determine whether gems are genuine. 

The Fluorescent Mine

OK, that’s some technical stuff out of the way, from a non-expert on the subject, and a short list of common minerals that exhibit fluorescence. So now for something completely different; instead of looking at individual minerals and their fluorescent properties, we are going to start by looking at a whole mine that fluoresces! For this we are going to the Sterling area of New Jersey in the USA, which has been dubbed the Fluorescent Capital of the World.

The mineralogy of the Sterling Hill and Franklin ore body is unique in that the main zinc ore minerals are franklinite (zinc-manganese-iron oxide), willemite (zinc silicate) and zincite (zinc oxide). The two mines have yielded about 30 million tonnes of ore over two centuries of mining, with the Sterling Mine closing in 1986. However, the area is still very much open and popular with scientists, rock-hounds and the general public in the form of the Sterling Hill Mining Museum at Ogdensburg (https://www.sterlinghillminingmuseum.org/warren-museum-of-fluorescence).

These visitors (c.40 000 a year) are not only interested in the mining history of the area and the unique mineralogy but also the colourful fluorescence exhibited by both individual mineral specimens and the spectacular fluorescence exhibited by the remaining in situ ore and associated minerals in the Sterling Hill Mine.

Figures 2 to 5 illustrate some of the wonders to be seen at the site, showing various faces, tunnels and stopes of the Sterling Mine under short-wave UV light, with calcite being orange and red, willemite green and hydrozincite blue.

Figure 2 - Stirling Hill mine rainbow room


Figure 3 - Stirling Hill mine tunnel


Figure 4 - Stirling Hill quarry wall


Figure 5 - Stirling Hill stope

Figure 6 shows a small selection of Sterling Hill minerals under UV light from the Thomas S Warren Museum of Fluorescence, while Figures 7 and 8 show typical Sterling Hill mineral groups of willemite and calcite under normal and short-wave UV light. In Figure 8b the franklinite shows up black as it doesn’t fluoresce. The current list of fluorescent minerals found at Sterling is 89.

Figure 6


Figure 7 - Willemite and calcite under normal UV light


Figure 7b - Willemite and calcite under short-wave UV light


Figure 8


Figure 8b - the franklinite shows up black as it doesn’t fluoresce

Concluding Remarks

This is a very short article that barely scratches the surface of the subject and has a fairly narrow focus both site and mineral-wise. However, if, like me, you weren’t previously “into” fluorescent minerals, hopefully, like me, you will be now.

References

The Mineralogical Record. (2016). Mineral Collections in the American Northeast. Supplement to The Mineralogical Record, July-August 2016. Tucson.

Fisher, J. et al. (2006). Fluorite: The Collector’s Choice. Lithographie. Connecticut.

Staebler, GA and Wilson, WE. Eds. (2008) American Mineral Treasures. Lithographie. Connecticut.

Heritage Nature and Science Auction. (2014). Fine Minerals, Gems and Lapidary Art. Catalogue, September 28. Dallas.