Fred, you are trying to explain something which you will probably not fully understand for years to come: Colour and transparency relate at least in part to the electronic band structure of the material, which in turn is explained via quantum mechanics. In short, a transparent material is one which does not absorb light in the visible region of the electromagnetic spectrum. Or to put it another way, the energy of the incoming light does not correspond to any available energy by which the material can enter an excited state via absorption of the light.
Once you have a material which does not absorb visible light you still need some other things to be true for it to be transparent. If it is crystalline it needs to exhibit a low number of crystal flaws, no cracks, and it is implicit that it should be a single crystal. The presence of crystal flaws and cracks scatters the light within the crystal, which causes the otherwise transparent material to become white (grind up some glass to see this effect).
As for the structure. CaF2 is the mineral \"Fluorite\" and the crystal structure is named after it. It is cubic, not hexagonal. As a point of interest, many other compounds also crystallise with the fluorite structure, not just CaF2.
Nov 08, 2009 at 05:02 PM
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