OTHER CAUSES OF COLOR IN MINERALS
Band gap processes cause color in gems!
This topic is not required for students of EPS2.
relevant to metals and semiconductors:
delocalized electrons -> energy band:
- -CB conduction band, higher E
- -VB valence band, lower E
Difference between energy of CB and VB is the 'band gap'
If this energy difference is within the range of visibl e light, it can produce color in gems...
- E band gap >> E visible light :- colorless (insulators)
- E band gap << E visible light :- all light promotes transitions, all l ight absorbed, thus black. If all light released again, -> metallic luster !!
- E band gap is approx. E blue light -> red mineral
- E band gap is approx. E red light -> blue mineral
Impurities: may introduce alternatives, additional destinations within the band gap:
PHYSICAL SUBDIVISION CAUSES COLOR IN GEMS!
This topic will be touched on briefly later in the course. It is not required, e xcept as it is relevant later to feldspars and opal
Opal is made of spheres of amorphous (non-crystal
line) material.
The regularly arrange
d spheres have a size comparable to the wavelength of visible light.
The color is caused by diffraction of light by the pattern of spheres. The resul
t is a very pure color (mostly blues and greens) in each patch with specific siz
ed spheres.
