A brief history

amenez, mthoraval — 2014-01-30T12:10:45Z

In the 1930's, quite by chance, mineral prospectors noticed that certain rocks changed colour when exposed to filtered spectrum of a mercury lamp (a source of black light). During the Second World War the ultraviolet lamp gained importance. The U.S. Army realised that tungsten was of strategic importance for army equipment. However tungsten deposits had not yet been discoverer in the U.S.A.

Then large finds were made in the old mining regions. All the miners had to do was to switch on their ultraviolet lamps for the blueish-white colour of the tungsten ore (scheelite) to show. This is how the U.S.A. became the world's major producer of tungsten during World War Two.

After the war, the search for uranium gave new impetus to the ultraviolet lamp. It was used by miners to differentiate between uranium and thorium, and in smaller mines to detect the zones richest in ore.

Some deposits were discovered purely by chance. A ranch owner in Nevada was going down into an old gold mine with his lamp when to his surprise the galleries lit up in a yellowish-green colour, the fluorescent colour of Autunite. A large deposit had been found. Thereafter, prospectors used ultraviolet lamps to search for deposits of lead, zinc and fluorite.

Why are certain minerals fluorescent ?

amenez, mthoraval — 2014-01-30T12:04:14Z

Sunlight - known as “white light” - corresponds to a series of different lights that can be obtained through an optical prism. These lights are the colours of the rainbow and each of them has a different wavelength.

Our eyes can identify the wavelength corresponding to the colours red, orange, yellow, green, blue, indigo and violet. For example, the leaves of a tree absorb most of the light emitted by the sun, except one. This wavelength is reflected to our eyes and we identify it as being green. Thus the colours we see in daylight are reflected colours.

Fluorescence is not a reflection of ultra-violet lamp rays. Fluorescent colours come from the mineral itself. They depend on its physical structure and on its chemical composition.

When it reaches the mineral, the energy of the ultra-violet ray displaces electrons. This movement releases electric and magnetic energy and is characterized by a ray of light. The phenomenon is instantaneous.

Fluorescence is often due to the presence of impurities, known as activators, in the mineral. These activators, such as manganese, copper, lead, silver, uranium, rare earths and water molecules, produce different colours. Futhermore, the great majority of fluorescent minerals only react to a particular ultraviolet range. Fluorescence depends not only on the mineral itself, but also on the lamp used - long waves or short waves.

ExpOOs / Maison des Minéraux

A brief history

Why are certain minerals fluorescent ?