|North America||San Rafael Swell, Emery County, Utah||9" x 7 ¾" x 2"|
Celestite (officially nowadays, celestine) is SrSO4, a sulfate mineral. Strontium (Sr) is not a common chemical element, but it has some uses in modern society, and celestine is the principal source of strontium. Strontium compounds provide the brilliant reds of fireworks displays. Have you ever seen glow-in-the-dark paints or plastics? A strontium compound again. On the negative side, radioactive strontium is produced as a byproduct of nuclear reactors.
There is another mineral, barite (BaSO4) that has the same crystal structure as celestine, and several studies suggest a solid solution series between the two. That means that any composition between BaSO4 and SrSO4 can occur, such as Ba0.6Sr0.4SO4. Of course, there are no such things as fractional atoms, so this formula just means that this hypothetical example contains 60% Ba and 40°/o Sr atoms.
As it happens, the great majority of barite-celestine specimens analyzed are within about 15% of the end-member compositions - either barite or celestine. Where are the intermediate compositions? One possibility for their apparent absence is that the sizes of Ba and Sr atoms are not close enough to permit complete interchangeability in the crystal structure. Another is that the intermediate members occur only under certain geologic conditions, and specimens from such environments have not been examined. Another is that typical geologic environments are either Ba-rich or Sr-rich, but not both. Science would be boring if all questions had been answered already!