Why do some mineral fluoresce?

First of all, let's deal with UV light. UV (Ultra Violet) light is past the blue end of the visible light spectrum. They have a wavelength of around 4x10^-7 to about 5x10^-8m. Figure one shows the visible light spectrum, with UV light to the right of this.

	Figure 1: The Electromagnetic spectrum. UV radiation is highlighted.

Fluoresce is caused by electrons changing energy states and then falling back to a lower energy level, emitting light as they do so (fig 2). In order to move the electron in the first place you need to input energy. UV light provides this energy on the case of phosphates. This is why white clothes glow in UV light - the phosphates in washing powder fluoresces. You do need some energy at the right frequency in order to make the electron jump levels, so you do need UV light - without it, phosphates would not fluoresce.

	Figure 2: The emission of light by electrons switching energy levels.

There are also many minerals that fluoresce, although it is an unpredictable property. Fluorite is the only mineral to receive its name from fluorescence, although not all fluorites fluoresce! Other minerals that fluoresce are calcite, willemite and diamond.

Come and discuss this and other topics in the forums.

References

C. Klein and C.S. Hurlbut, Jr, 1993. Manual of Mineralogy (21st Edition).

D. Halliday, R. Resnick and J. Walker, 1997. Fundamentals of Physics..