If the overgrowth crystals have a similar orientation there is probably an epitaxic relationship, but it is not certain.

Minerals that have the same composition but different structures (polymorphic minerals) may also have epitaxic relations.

It is easy to understand that minerals that have the same structure (isomorphic minerals) may have epitaxic relations.

If the crystals of both minerals are well formed so that the directions of the crystallographic axes are clear then the epitaxic relationship can be deduced just by a visual inspection.

Some authors consider that overgrowths of a second generation of the same mineral species should also be considered as epitaxy, and this is common terminology for semiconductor scientists who induce epitaxic growth of a film with a different doping level on a semiconductor substrate of the same material.