I had an opportunity to travel to Philadelphia at the end of last week on business, and I found it an opportune time to travel north a bit on Saturday to Franklin, New Jersey to one of my favorite places . . . the Franklin Mineral Museum and the fluorescent mineral collecting site directly behind the museum. This was my fourth trip to Franklin and I enjoy it more each time. I was looking for flourescent minerals and I sure found a load of them on Saturday. I just happened to be there on the first day of the collecting season. A week earlier and I would have found the site under snow.
The first rock that I spotted looked different to me. I didn't recall having seen one like it there before, so I held on to it and kept moving it to the top of my bucket each time I placed another rock in the pile to be sorted. My first rock turned out to be a combination of willemite, zincite and franklinite. When I placed it under the short-wave UV light, it fluoresced a bright blue. It is really stunning. I also found several red willemite and several green willemite specimens. I found a piece or two of barite, there were several specimens with just a tinge of yellow fluorescence and one or two with specs of beige. And of course, there was "tons" of calcite brilliantly glowing orange. I have always found the combination of calcite and franklinite to be an attractive-looking specimen. The pictures above are red willemite from my Franklin excursion. The top one in under fluorescent light. The bottom one is under short-wave UV light.
Now just a few words about fluorescence.
Fluorescence is a property not found in all minerals. Minerals that do fluoresce, glow when exposed to either short-wave or long-wave ultraviolet light. Examples of fluorescent minerals are autinite, calcite, diamond, eucryptite, fluorite, hyalite, scheelite and willemite. Minerals from the Franklin and Sterling Hill area of New Jersey are known for their fluorescence. There are over 80 fluorescent minerals found in that area. Franklin/Sterling Hill fluorescent specimens usually contain 2-4 minerals in a typical specimen though some have up to 7 fluorescent minerals found together.
Fluorescent minerals contain particles in their structure 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. The reaction will only be visible in a dark area, where the presence of white light is weak.
There are two ultraviolet wavelengths: long-wave and short-wave. 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 long-wave and the other for short-wave. There are more minerals which fluoresce in short-wave than there are in long-wave.
Color and intensity of the fluorescence varies among specimens of a particular mineral. However, specimens from the same locality almost always fluoresce the same color. For example, calcite may fluoresce red, orange, yellow, white or green.
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.
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