pseudo-symmetry

from The Century Dictionary.

• noun In crystallography, false symmetry; the appearance of having a higher degree of symmetry than is actually the case, usually produced through twinning. See twin.
• noun A crystal exhibiting pseudosymmetry is often named according to the system of symmetry which it imitates (through twinning, distortion, etc.) as follows: pseudisometric, pseudotetragonal, pseudohexagonal, pseudorhvmbohedral or pseudotrigonal, pseudo-orthorhombic, pseudomonoclinic, etc.; for example, complex twins of phillipsite sometimes resemble isometric dodecahedrons and are hence said to be pseud-isometric; also, isometric crystals (as of gold, silver, and copper) by symmetrical distortion, that is, by flattening or elongation in the direction of an octahedral (trigonal) axis, are often strikingly pseudorhombohedral.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Crystallog.) A kind of symmetry characteristic of certain crystals which from twinning, or other causes, come to resemble forms of a system other than that to which they belong, as the apparently hexagonal prisms of aragonite.