Amethyst ( #4)
|North America||Jackson's Crossroads, Wilkes Co., Georgia, USA||3 ½" x 2 ½" x 1 ¾"|
Amethyst is the purple variety of quartz - the same mineral as the colorless quartz in the massive specimen that is the centerpiece of this display. So, you ask, why is amethyst purple? In fact, what causes color in any mineral? There are essentially four fundamental causes of color in minerals:
- Color centers
- Crystal field effects
- Charge transfer
- Electron band effects
All of these have to do with the interaction of light with electrons. When white light, which contains all of the colors (wavelengths) of the rainbow, strikes a mineral (or anything else), if some of the wavelengths are absorbed by electrons, then the color you see is what is left in the spectrum.
Amethyst owes its purple color to color centers. The chemical formula of quartz is SiO2, but sometimes iron (Fe) atoms in trace (exceedingly small) amounts replace some of the silicon (Si) atoms, or else they occupy places in the crystal structure not normally occupied by any atom. Natural radiation from surrounding rocks can sometimes knock an electron from an Fe atom. When light interacts with such an Fe atom, wavelengths in the middle of the spectrum are absorbed, leaving the long (reddish) and short (bluish) wavelengths in the light you see. Hence, the purple color.
Several minerals in this display show color caused by color centers. We'll explain how those and the other causes of color work in the screens for the appropriate mineral specimens.