Fluorescence and Minerals at El Fenomeno

Fluorescence

Stoke's Law: The light waves emitted by a fluorescing substance will be of a longer wavelength than those used to excite the fluorescence.

The most striking and useful application of this law occurs when invisible ultraviolet (UV) light is directed toward a suitable substance, which then emits longer wavelengths than those of UV, placing them in the visible portion of the spectrum and thus making the fluorescent display perceptible by the human eye. Conversely, it has been found that visible light can be used to excite a similar response in certain substances, but the longer wavelengths emitted by the substances fall in the infrared region of the light spectrum and are invisible to the eye (Sinkankis, 1972).

Ultraviolet Light

Useful range: 4,000 - 2,000 Angstroms (A)

• Long wave (LW) region: 4,000 - 3,150 A
• Short wave (SW) region: 2,800 - 2,000 A.

Most useful lamp emissions for UV work: LW - 3,660 A. SW - 2,537 A.

(above) Tactite under short wave UV
(below) tactite in visible light

Fluorescence In Minerals

The following discussion of fluorescent minerals is provided by the Fluorescent Mineral Society Inc. at the website, http://www.uvminerals.org/minerals.htm.

Well over 3600 mineral species have been identified at this time; over 500 of them are known to fluoresce visibly in some specimens.

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, notably ferrous iron, that can prevent fluorescence despite the presence of an activator. Because the activators and deactivators may or may not be present in any given specimen, different specimens of the some mineral (especially from different locales) may vary in color and degree of fluorescence. A common fluorescent mineral is calcite. It comes in just about all the observed fluorescent colors due to different activators. Red and pink fluorescent calcites are often activated by a combination of lead and manganese.

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 presence of 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.

Minerals at El Fenomeno

The observed fluorescence in the minerals at El Fenomeno to date is given in the following table in bold. The other colors listed for the mineral have been noted by Sinkankis in the Gemstone and Mineral Datebook (1972); the frequency is given as Common (C), Occasional (0), or Rare (R).