calcite - hot pink fluorescent and phosphorescent calcite cleavage rhomb from the Challenger Cave system, Nuevo Leon, Mexico

Calcite, CaCO_3, is an extremely common rock forming mineral. In pegmatites it forms large masses and it makes up some limestones almost entirely. It has a large variety of crystal forms, but these do not include the rhombus-shaped pieces that are sold as “calcite crystals.” The rhombs are produced by cleaving calcite in three directions between rows of atoms. 

When calcite is free of impurities and is optically clear, it exhibits double refraction. When it is placed above a line drawn on paper and rotated, the drawn line doubles and becomes single depending on the angle of rotation. These are not clear enough to show double refraction.

Under longwave ultraviolet, these specimens fluoresce hot pink. The first photo shows this fluorescence. The second photo compares the hot pink fluorescing calcite with some from the same locality that fluoresces a cooler pink. The third photo shows what to expect under shortwave, though it may be paler than what the camera picked up. These can have a bluish-white or mottled shortwave response as well, with all showing excellent phosphorescence that is fairly slow to extinguish.  

In a phosphorescent substance, electrons that absorb energy slowly re-emit it, sometimes glowing for minutes or even hours as they return to their original energy states. Most phosphorescent compounds are fast emitters, however, with lifetimes on the order of milliseconds. You can watch this calcite extinguish as the atoms return to their normal energy state after exciting it with shortwave for a few seconds.

These clean sharp rhombs of colorless calcite are from the Challenger Cave system near Monterrey, Nuevo Leon, Mexico, a famous locality for fluorescent calcite. 

If tiny rhombs would be useful in your classroom, let us know and we'll include some, though not of this calcite, for student experimenting and so you can demonstrate the calcium carbonate response to a drop of hydrochloric acid - it fizzes and releases carbon dioxide - without wrecking this example.

You will need a strong and well-filtered shortwave lamp to see the bluish fluorescence and the phosphorescent decay after the lamp is extinguished. We can recommend The Triple, sold by Way Too Cool, LLC at https://www.fluorescents.com/products-the-triple.html. It will give you short, medium  and longwave ultraviolet.

For the hot pink fluorescence, any ultraviolet flashlight - they are all long wave - will work. Way Too Cool has those as well.

Click on an image to enlarge and click again to further enlarge. The pen is 5" long, for  scale. The pencil is 5 1/2" long.

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil (it has an eraser), or one of those plus some number of coins, or you can let us make the selection.

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