WHAT IS FLUORESCENCE?

Fluorescent materials emit light when struck by light of a higher frequency (shorter wavelength).

Incident light excites electrons in the fluorescent material which, in returning to their ground state, emit light. This light is known as fluorescence.

Of interest here is the case in which the excitation light source is invisible ultraviolet (UV), and the emitted light is in the visible range (VIS). The results, when suitable mineral specimens are placed in darkness and bathed in invisible UV, can be stunning.

Otherwise dull looking rock suddenly bursts into colored light as if by magic. The light is cold to the touch, but the rocks themselves are truly glowing—they are luminous minerals.

Ultraviolet exists in a range of wavelengths, just like the familiar red-orange-yellow-green-blue-violet of the VIS spectrum. All UV is invisible to humans, though some bees and other insects can see in part of its spectrum. To cause minerals to fluoresce, collectors generally use UV lights emitting in one of three portions, or ‘bands’, of this spectrum: longwave (~ 365 nm), midwave (~ 300 nm), or shortwave (254 nm).

Fluorescent minerals at Ilímaussaq may respond to one, two or all three of these wavelength ranges, but each species is generally brighter under one than another. In at least one case, a variety of tugtupite, samples may emit a different colored light under each UV band.

Roughly 10% of mineral species worldwide are known to fluoresce, but their response is often dim, and variable. One sample may fluoresce, and another of the same species but from a different locale, may not.

In a few exceptional localities, such as the long famous zinc deposits at Franklin and Sterling Hill, New Jersey, a large variety of brightly fluorescing minerals are found together. In such places ‘combination’ samples are possible—several species glowing at once in the same hand specimen; each in their own distinctive color.

These can be beautiful when viewed under the right UV source(s). Based on our recent explorations Ilímaussaq can easily be added to a very short list of exceptional fluorescent mineral localities.

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