Photoluminescence is a process in which a chemical compound absorbs light energy, thus jumping to a higher energy state. When the light energy is released, it returns to a lower energy state. Two common types of photoluminescence are fluorescence and phosphorescence.
When atoms are exposed to ultraviolet light (UV or “black” light), the photons (light energy) cause the electrons (subatomic particles) to move into a higher energy state. The immediate release of light energy that is seen is called fluorescence. When the ultraviolet light source is removed, the object does not continue to fluoresce. Phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light causing an afterglow. The electrons get ‘trapped’ in the higher energy state for a little while and light energy is released at a slower rate. The wall next to the Surprise Cave glows for less than 10 seconds, or so. When the electrons escape the ‘trap,’ they fall back into their natural state and the object stops glowing.
Cave of the Mounds has calcite formations and we demonstrate its fluorescence AND phosphorescence every day. However, the formations do not glow because they are made of the mineral calcite. Calcite may or may not fluoresce. The easiest way to explain this process is to first discuss the make-up of calcite. All minerals are made up of elements. When a mineral forms, there are usually impurities present which may or may not be in the normal molecular structure of the mineral. Pure calcite does not exist in nature. In a lab, all impurities may be blocked and pure calcite can be created, but nature does not work that way. The impurities present in the environment where calcite forms play an important role in whether or not the calcite will fluoresce. Calcite, pure CaCO3, does not have an excited state and therefore would not fluoresce all by itself. Impurities sometimes serve as activator elements for photoluminescence. The activator element manganese (Mn+2) in the matrix of our calcite causes the fluorescence that is witnessed as the afterglow on the wall beside the Surprise Cave.
A great analogy– If you think of minerals as a classroom of kids, they will exhibit their normal energy state in the classroom. When they go outside for recess, they (like the electrons in an atom) get into an excited state. This is like fluorescence. There are still those that remain wound up after returning to the classroom when most of the class has calmed back down. This is like phosphorescence.