pseudo-symmetry

from The Century Dictionary.

• 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.
• 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.

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.