Fluorescence is a light-generating phenomenon in which a substance is excited by light of one wavelength and subsequently emits light of another, generally longer wavelength.

The most frequently used light source for causing fluorescence in minerals is ultraviolet, and the longer wavelength light emitted is in the visible portion of the electromagnetic spectrum. 

Since ultraviolet light is invisible, the visible (fluorescent) result may appear as if by "magic" when viewed in a darkened room.

Less than 15% of the roughly 4,000 species of minerals fluoresce under ultraviolet, and of those that do a much smaller number does so consistently or brightly. Ilímaussaq has a variety of vividly fluorescent mineral species.

ULTRAVIOLET
Ultraviolet includes a broad range of wavelengths—from roughly from 400 nm to 200 nm. The ultraviolet spectrum is bordered at its 400 nm end by the most violet light we can see; beyond its 200 nm end by the longest wavelengths of X-rays.

 
Partial electromagnetic spectrum; scale in nanometers

By convention, the ultraviolet spectrum is broken into arbitrary divisions: longwave or "UVA" (~400—350 nm); midwave or "UVB" (~350—300 nm); shortwave or "UVC" (~300—200 nm); and far ultraviolet (<200 nm).

A particular fluorescent mineral may respond best to ultraviolet in a certain range of this spectrum; some will respond in different colors depending on the excitation wavelength.

ULTRAVIOLET LAMPS
The principal ultraviolet light source in use today is based on light emission from ionized Hg vapor contained in a low-pressure glass or fused silica tube (photo at left).

As atoms of mercury within these lamps are excited they emit a very strong light at 253.7 nm (shortwave ultraviolet). This may be passed through the walls of a fused silica tube (shortwave ultraviolet does not penetrate glass) and then through a special dark filter, providing a lamp rich in 254 nm shortwave ultraviolet.

Other lamps can be made with varying formulations of phosphors coating the inside of the tubes. These absorb the 253.7 nm ultraviolet and fluoresce in other regions of the ultraviolet spectrum, depending on the phosphor. In fact, common household ‘fluorescent lights’ are based on similar technology. In that case, however, the tube is glass opaque to ultraviolet and the phosphors inside fluoresce in various parts of the visible spectrum—combined they appear as white light.

Mineral collectors employ lamps in the shortwave, midwave and longwave regions of the ultraviolet; far ultraviolet does not pass through air very well and is both dangerous and impractical.

Most shortwave lamps in use today emit almost completely at 254 nm (as below); longwave sources have more variable outputs dependant on phosphor mix and filtering.

The two longwave sources we use have tubes with peaks at 352 nm and 368 nm (upper right), but other wavelengths are present as well.

Several midwave or "UVB" lamps have become available in recent years, and these also have outputs dependant on phosphor mix and filtering.

The one used for midwave photographs on this website has the spectrum shown at right, which includes a fairly broad range of wavelengths.

 

ILÍMAUSSAQ FLUORESCENCE
To best experience the fluorescent minerals of Ilímaussaq one should have access to lamps in all three ultraviolet regions. The local sodalite responds most brightly to longwave or midwave ultraviolet; while associated fluorescent minerals may glow best under shortwave. The tenebrescent effect in both sodalite and tugtupite is stimulated by shortwave ultraviolet. Tugtupite glows in different colors under differing ultraviolet sources. Some responds with a unique white color under midwave only.

A large display of Ilímaussaq fluorescent minerals would best have a ‘full spectrum’ of ultraviolet light sources: short-, mid-, and longwave.

ACTIVATORS
Some minerals, like scheelite, are intrinsically fluorescent—their ideal chemical formula and perfect crystal structure provide a fluorescent material. Most fluorescent minerals however owe their glow to the presence of minor/trace impurities or structural defects. Hence, the fluorescence of many species is variable; depending on a few ‘alien’ atoms or structural ‘holes’.

At Ilímaussaq, the late-stage lujavrites are enriched in volatiles—including S2-. The presence of minor amounts of sulfur has been identified as the cause of fluorescence in both sodalite and tugtupite.

 

  
 
 
 


This page edited: April 23, 2005.    SimpleThinking