Fluorescent Minerals

Well over 3600 mineral species have been identified at this time. Something over 500 of them are known to fluoresce visibly in some specimens. FMS members have assembled a list of web sites showing examples of fluorescent minerals and a database of locales of fluorescent minerals.

Most minerals do not fluoresce when pure. It takes certain impurities in certain quantities to make the mineral fluoresce. Such impurities are called "activators". Different activators can make the same mineral fluoresce in different colors. Different minerals require different activators, and in different quantities. There are also impurities called quenchers, notably ferrous iron, that can prevent fluorescence despite the presence of an activator. Because activators and quenchers may or may not be present in any given specimen, different specimens of the same mineral (especially from different locales) may vary in color and degree of fluorescence.

There are a few minerals that will fluoresce when pure. These are called "self-activated" minerals, and include scheelite, powellite, and several uranium minerals. Others suspected of being self-activated include benitoite, cerussite, anglesite and perhaps many other lead minerals.

Scheelite, a major ore of tungsten, is often found by its brilliant sky-blue fluorescence. If it has a little molybdenum in it (which makes it troublesome to extract the tungsten), this color is modified to white or yellow, providing a quick way to assess the commercial value of a find.

Several secondary uranium minerals, such as autunite, are also characteristically fluorescent a bright yellowish green. This is due to the uranyl ion; this ion is so prone to fluorescence that trace amounts of it cause yellowish-green fluorescence in a very large number of minerals, including adamite, apophyllite, aragonite, calcite, quartz, and opal. Any yellowish-green fluorescence other than willemite is likely to be due to the uranyl ion.

A common fluorescent is calcite.   It comes in just about all fluorescent colors due to different activators. Red and pink fluorescent calcites are often activated by a team of lead and manganese. Calcite may fluoresce green due to uranyl ion traces. Calcite from the mercury mines at Terlingua, Texas, and just across the border into Muzquiz, Coahuila, Mexico, is a favorite. It fluoresces pink under longwave UV and bright blue under shortwave UV, with a uniquely bright blue phosphorescence after the UV lamp is removed.

Fluorite gave its name to fluorescence, but that does not mean it is always fluorescent. Many fluorites fluoresce a blue-violet color due to traces of europium; this is usually best under longwave UV. Fluorite shows other colors of fluorescence in some cases.

Willemite, a zinc mineral, is often fluorescent a bright yellowish-green, due to traces of manganese.

Scapolite (wernerite) from Ontario and Quebec, Canada, fluoresces a vivid orangeish-yellow color under longwave UV, while shortwave UV inspires a long-lasting phosphorescence that can be markedly brightened by holding it under a running hot water faucet, illustrating thermoluminescence. Several other fluorescent minerals come from the same area, including sodalite (hackmanite), cancrinite, diopside, fluoborite, and nepheline.

Franklin, New Jersey is rightly known as the "fluorescent mineral capital of the world". Together with nearby Ogdensburg, it is the source of at least 260 minerals, of which at least 56 are fluorescent.   Many of these minerals are found nowhere else in the world. Many of the fluorescents are uncommonly bright. Only the Franklin mine provided specimens with 5 or more different colors of equally bright fluorescence from as many minerals in one specimen. Willemite and calcite specimens, glowing yellowish-green and orange-red, respectively, are the most common fluorescents from this area; sometimes these combine with hardystonite (fluorescing deep blue-violet), clinohedrite (fluorescing orange), and maybe even esperite (fluorescing yellow) for a truly unforgettable rainbow fluorescence.

Unfortunately, both the Franklin and Ogdensburg mines have been closed. The Sterling Hill mine at Ogdensburg, though the deep levels are flooded, has recently reopened as a museum. The surrounding marble formations still supply many fluorescent specimens, but no more of the rarest combinations are found.

Potential Exploration Links