Is every space group realized by some mineral?
Now all of the 230 space groups are realized by a natural occurring mineral.
I found this blog: crystalsymmetry.wordpress.com where space groups are listed with at least one chemical compound is listed for each space group. Minerals are listed for all of the space groups.
For example, Benitoite has fairly unusual crystal symmetry. I think it was first mineral found for P-6c2 space group.
188 P-6c2 Example – Benitoite, BaTi(Si3O9)
From Crystal Structure of Benitoite.
From a crystallographical point of view this compound (Benitoite)
is especially interesting, being the only known member of the ditrigonalbipyramidal
symmetry class.