These minerals, primarily zircon, rutile, monazite, and xenotime had been washed out of the ancient Appalachian mountains long ago. The eastern mountains of North America are very old and were, in their youth, much larger than the Rockies. They may have been as tall as today's Himalayas. To be worn down to their present size, a lot of stuff has been washed out and some of it was deposited near the ocean in Georgia.
The sand was scooped up and washed down helical troughs. As it descended the troughs, it moved faster and faster spinning off, first, the lighter sand and, then, heavier and heavier materials, separating the sands nicely by density. Our guide gave us four very generous samples.
The sand was very fine but our microscope revealed wonders. The hard minerals retained their crystal forms, tiny sparkling jewels. The zircon provided the extra of being fluorescent, glowing a dull red in the light of our ultraviolet lamp.
When light meets matter, some of it is absorbed and several things can happen according to the intensity and wavelength of the light, and the kind of matter. The vibrations of infrared and microwave radiation are too big to get into the atoms that make up most materials, but they can certainly shake up the larger particles. That's why infrared and microwave radiation heats things up. Heat is just the motion of particles that make up matter. Shorter wavelengths can move the electrons around in individual atoms and make some interesting things happen.
When visible light hits an atom, the energy it imparts can kick an electron further out from the nucleus of it's atom. When it falls back to it's rest state, it will give the energy back as the same wavelength of light (in a colorless material) or as some other wavelength of light (in a colored material).
There are shorter wavelengths of light. Most people can't see ultraviolet light (aka "black light") but some substances can absorb it and re-emit light of a visible wavelength giving the impression that the material is glowing with its own light. Designers use this phenomenon, called "fluorescence", to make highly visible objects. The terms "dayglow" and "neon" are often used to describe the products. "Dayglow" is especially appropriate since there is enough ultraviolet light in sunlight to set them off and give them a vibrant appearance.
Ultraviolet light is often divided into two categories - long and short wavelengths. The longer wavelengths are close to the visible spectrum. In fact, some people can see some of the longer wavelengths of ultraviolet light. These are the wavelengths targeted by designers because shorter wavelengths...
well, they burn you. Short wavelength ultraviolet light is the part of sunlight that causes sunburn. The shorter the wavelength of light, the more energy it carries, and the larger a push it can give electrons in atoms. Short wavelength ultraviolet is the beginning of the spectrum of ionizing radiation. It is so energetic that it can knock electrons right off atoms. Shorter wavelengths include X rays and gamma rays.
I remember LED lights when they became available to consumers. The Christmas lights were laughable. At that time, you could only get red, green, and yellow LEDs but they were very dim. Later they figured out how to make them much brighter. But the goal was LEDs for illumination because solid state LEDs operate cooler, last longer, and use way less electricity to produce the same amount of light. The problem was...who wants to live in a house lit by red, green, or yellow lights?
LEDs are near-monochromatic light sources. One will only produce light in a very narrow band of colors. After a while, substances were found that emitted all the colors, from infrared to ultraviolet, but white, the color most people want to live in, isn't actually a color. It's a mixture of all the colors.
Engineers found two solutions. When they placed different colors of LEDs together they could mix the colors to give off a reasonable illumination. Then, the problem became a simple matter of aesthetically balancing the color blend.
The other solution became the most common. Here's a picture of the ceiling lamp in my bedroom.
Here it is turned on.
Those are LED bulbs. When I shine my ultraviolet flashlight (with it's single ultraviolet LED) on one of the bulbs, it looks like this.
That's not terribly impressive, but it is white. The secret of the white light LED bulb is that the light element is a printed circuit board with several ultraviolet LEDs plus some other color LEDs to balance the color produced. And the inside of the bulb is coated with a fluorescent material.
The bulbs are well designed and not a lot of the UV light escapes. I know this because even the long wavelengths make my skin tingle. Also, I have some neon paints and they look pretty drab under the house lights.
Sunlight perks them up and the UV flashlight really makes them pop.
Recently, in a dollar store I noticed a disinfectant wand for less than $10. These things put out weak, shortwave UV light, so I bought one. Fluorescence is pretty specific. A material than will glow under one wavelength might not under another, or they might emit a different color. Here's what the paints look like under the short wavelength UV.
The glow is dim but, remember, the wand puts out much less radiation than the flashlight. Still, the colors of the fluorescence are about the same except the blue paint doesn't fluorescence at all under this short wavelength UV.
In 2000, I attended the PathwaysToDarkness party in Atlanta, Georgia. It was a meeting of vampires and weres. Appropriately, it was held at night and the place was dark (some vampires are sensitive to daylight and burn easily.) Except for the bar. There was a blacklight. And, of course, there was tonic water and absinthe. Why, "of course"?
Well, they both fluoresce under blacklight. Absinthe glows green and the quinine sulfate in tonic water blazes a brilliant blue. Want to see it?
Tonic water, for those that don't drink, is a bitter component of various mixed drinks that contains quinine sulfate measured in parts per million, so the glowing substance is present in only a very tiny amount. It became popular in the 1800s when it was found to have a curative affect on malaria. My father, who contracted the disease in southern Georgia began drinking again in old age to ease his leg cramps.
It doesn't fluorescence at all under my disinfectant wand but there are short wavelengths that do cause fluorescence.
The main fluorescent entity isn't the quinine sulfate but the ions, quinine hydrogen sulfate and quinine dihydrogen sulfate. For that reason, increasing the hydrogen ion concentration of tonic water increases the fluorescence. Tonic water that uses carbonated water as solvent is already slightly acidic but I added some lemon juice to my tonic water to get a blazing blue glow.
Many things glow under black light making the ultraviolet flashlight a useful instrument for field geology. Scorpions, for instance, give off a ghostly white light. Some minerals also give off a characteristic glow and can aid in night prospecting.
When I was collecting, I had a few minerals that would glow brilliantly under a black light. A few were from Franklin, New Jersey, "The Fluorescent Mineral Capital of the World". The zinc mines around there produce excellent specimens.
The few minerals I have now are so-so.
Fluorescence is named for the second minerals from the left and right in the top row, fluorite, which is calcium fluoride. The one to its left also commonly fluoresces. It's calcite, the mineral that makes up limestone and marble. The mineral spotlighted on the bottom row is also calcite.
Caves generally form in limestone and the eastern United States is practically a limestone sponge with the Appalachian mountains floating on it. There are a lot of caves.
One favorite trick of cave tour guides is to carry a group deep in a cave and turn off the lights to show what darkness really is like.
I visited a spectacular cave in northern Alabama called Cathedral Caverns and they apologized. When they turned off the lights, all the rocks glowed dimly. That's called "phosphorescence".
In fluorescence, the atoms exposed to one wavelength of light absorb the light but immediately re-emit it with another wavelength. In the specimens above, invisible ultraviolet is re-emitted as red light. But in phosphorescence, it takes longer for the electrons in the atoms to calm down, so they are re-emitted over a period of time.
These particular minerals don't do much of anything under the disinfectant wand.
You can't see it in the photo, but the fluorite, second from the right in the upper row, does show a dull brown when exposed to this wavelength.
Sedimentary rocks, while they are often great for fossil hunters, do not usually provide good opportunities for mineral collectors, although some areas have gorgeous calcite crystals, but anywhere on land is an exciting place for a night hike with an ultraviolet light. Ultraviolet flashlights are inexpensive and even more powerful battery powered lights with both long and shortwave light sources can be found for less than $50.
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