Geometry of anisotropic double crystals

An anisotropic double crystal (double in short) is a mathematical model of simplest multi-crystal. It consists two connected regions the space enclosing prescribed volumes that contact each other. Its energy integral an density over surface, and depends on normal direction surface at point. A critical point this energy. We derive geometric characterization crystals. In plane, for certain function, we classify crystals terms symmetry given areas.