Fluorescence and Phosphorescence

Minerals that become luminescent during exposure to ultraviolet light, x-rays, or cathode rays are fluorescent. If the luminescence continues after the exciting rays are shut off, the mineral is said to be phosphorescent. Phosphorescence was early observed on some minerals that, after exposure to the sun, would glow on removal to a dark room. There is no sharp distinction between fluorescence and phosphorescence, for some minerals that appear only to fluoresce can be shown by refined methods to continue to glow for a small fraction of a second after the removal of the exciting rays.

Consequently, the phenomena are considered by some to be the same. More minerals show fluorescence than other types of luminescence, and since it is easier to produce, much work has been done on it. Fluorescence is shown by some fluorite, the mineral from which the property receives its name. Other minerals that frequently, but by no means invariable, fluoresce are willemite, scheelite, calcite, scapolite, diamond, hyalite, and autunite. Fluorescence is a property that cannot be predicted, for some specimens of a given mineral will show it, whereas other, apparently similar, specimens will not.

Not only do the fluorescent colors of different minerals and different specimens of the same mineral vary greatly, but also they bear no relation to the natural color of the minerals. A well-arranged display of fluorescent minerals makes a beautiful and striking exhibit. Fluorescence is most commonly produced by excitation with ultraviolet light, and each year sees improvements in the methods of producing such light. It is best to have a source of ultraviolet light with a minimum of accompanying visible light so that the fluorescent effects will not be lessened by reflection.

The iron spark, the mercury-vapor lamp, and the argon tube, or some variation of these, are most commonly used. The wave length or color of the light emitted during fluorescence varies considerably with the wave length or source of the ultraviolet light. Fluorescence is having an increasing commercial and industrial significance, and much work is being done on synthetic material. In connection with minerals, the property has a practical use at Franklin, New Jersey, where ultraviolet light is used to determine the amount of willemite that goes into the tailings.

Also, since most scheelite fluoresces, prospecting for that mineral is frequently carried out at night with the aid of ultraviolet light. Apparently no simple relation exists between luminescence of various kinds, for any kind may exist alone or with any of the others. Moreover, the color of the different types of luminescence may vary in the same specimen. Luminescence is rarely shown by pure compounds, and therefore one factor that may be common to all mineral showing luminescence is a small amount of impurity.

The wave length of the light emitted during fluorescence varies with the wave length or source of the ultraviolet light. Fluorescence is having an increasing commercial and industrial significance, and much work is being done on synthetic material.