Figure 6-8 is a simplified attempt to illustrate how a twin might form during the growth of a crystal. Which position is actually filled is normally determined by the equilibrium energy of the crystal in the two possible states. The energy minimum associated with site B is lower, and hence the normal equilibrium distribution would be to place the next layer in the B position. Energetically, site B is the most stable position for growth. However, if the energy difference between C and B is small, it is possible that an atom or ion may arrive at C rather than at B and the thermal agitation is not large enough to move the new atom or ion to the B position.
Twins can also form after a crystal has grown.
Figure 6-9 illustrates the possible formation of a transformation twin developing as a result of shearing of the crystal lattice. In this case shear in a N-S direction distorts the lattice and eventually results in glide . Note that in this case the twin plane is a glide plane. Twinning in Plagioclase Feldspar One of the most spectacular and yet common examples of twinning occurs in plagioclase. Plagioclase is triclinic and hence there are no planes or axes of symmetry to place restraints on the possible planes or axes of twinning.