Plesiotwinning: oriented crystal associations based on a large coincidence-site lattice

Oriented crystal associations with a low degree of restoration of lattice n odes are discussed and it is shown that they are based on a large coinciden ce-site lattice (CSL). In this type of associations the oriented crystals a re related by symmetry or pseudo-symmetry elements expressed by high indice s in the crystal lattice. The relative orientations of the crystals corresp ond to rotations belonging to non-crystallographic point groups. In spite o f the low number of common lattice nodes, these associations are not uncomm on, especially in layer compounds. Since this kind of associations does not enter either in the definition of twinning or in that of cell-twinning, we propose to introduce the definition of plesiotwinning. It is suggested tha t plesiotwins may form when two or more crystals coalesce or exsolve. A ple siotwin orientation may require a minor adjustment of the relative orientat ions, whereas a more stable configuration, such as a parallel growth or to a twin, may require a larger, kinetically more hindered rotation.