  |
Properties |
|
|||
|
|
||
 |
|
|
|
MINERAL PROPERTIES and identification procedures FLUORESCENCE and PHOSPHORESCENCE
Fluorescence is a phenomenon that causes a substance to "glow" in the visible spectrum when exposed to ultraviolet light. Minerals that exhibit fluorescence are known as "fluorescent minerals". Fluorescent minerals contain particles in their structure known as activators, which respond to ultraviolet light by giving off a visible glow. Ultraviolet light is a form of electromagnetic radiation invisible to the human eye. It is given off by the sun and by common fluorescent lamps used for lighting, but they also give off considerable white light (visible light), preventing the fluorescence from being seen. The ultraviolet reaction is visible with a special fluorescent lamp with a filter that blocks white light but allows ultraviolet light to pass through. This lamp is known as an ultraviolet fluorescent lamp, or UV lamp. Obviously, the reaction will only be visible in a dark area, where the presence of white light is weak. There are two classified ultraviolet wavelengths: longwave and shortwave. Some minerals fluoresce the same color in both wavelengths, others fluoresce in only one wavelength, and yet others fluoresce different colors in different wavelengths. Some UV lamps have two separate filters: one for longwave and the other for shortwave. Many more minerals fluoresce in shortwave than in longwave; only a fairly small amount of minerals fluoresce in longwave. Longwave fluorescent lamps are fairly inexpensive, whereas shortwave lamps can be costly. When this guide labels a mineral as fluorescent without specifying longwave or shortwave, shortwave fluorescence is implied. Fluorescence is not a reliable method for mineral identification, since certain minerals of the same species may fluoresce different colors from different localities. Once a mineral has been identified, its fluorescent color can sometimes be used to identify its place of origin. Similarly, if an unidentified mineral from a certain locality glows a specific color, it can often be identified by the color of its fluorescence. Color and intensity of the fluorescence varies among specimens of a particular mineral. However, specimens from the same locality virtually always fluoresce the same color. Calcite may fluoresce red, orange, yellow, white, green, etc., but is always red at Franklin, New Jersey, and bluish-white at Terlingua, Texas. When a fluorescent lamp is lit, never look at the light source, as it can damage the eyes permanently. In addition, skin should not be exposed to the light source for extended periods, as it can cause sunburns and long term skin problems. Phosphorescence is a phenomenon exhibited in several fluorescent minerals where the mineral continues to glow after the UV light source has been removed. The glow slowly fades, and after several seconds (or minutes in a few cases) is no longer visible to the eye. Only few minerals phosphoresce; phosphorescence is only noted as interesting mineral property, rarely as a diagnostic identification property. Certain mineral specimens from specific localities all phosphoresce. Calcite specimens from Terlingua, Texas fluoresce bluish-white with a strong phosphorescence.
|
