<unk>  <unk>  is an insoluble ionic compound of<unk>  and<unk> .  It occurs naturally as the mineral<unk>  (also called fluorspar), and it is the source of most of the world's<unk> .  This insoluble solid adopts a cubic structure wherein<unk>  is coordinated to eight<unk>  anions and each F- ion is surrounded by four Ca2+ ions.  Although the pure material is colourless, the mineral is often deeply coloured due to the presence of F-centers.
Calcium fluoride
Sodium fluoride
Stannous fluoride
Strontium chloride
Sodium monofluorophosphate
Magnesium silicate
Dihydroxialumini sodium carbonate
Mercuric iodide