Albite-law microtwins in plagioclase feldspars

The investigation by Fitz Gerald (1980) of microtwins in plagioclase feldspars is probably the most detailed yet undertaken of this type of defect in important rock-forming minerals and provides an excellent example of the theoretical and experimental procedures involved in characterizing these defects from their TEM contrast. 

Clearly, Rt depends on the structural state and composition of the feldspar concerned, through their influence on the lattice parameters. However, Rt is always in the (010) plane and has the form 

If the matrix only is diffracting, an cr-fringe pattern will be produced for which the phase angle a is given by 

Correspondingly, for a microtwin that is n unit cells thick, 

Values of u and w and of 2irg Rx for some low-index reflections g are listed for several plagioclase compositions in Tables 8.2 and 8.3, respectively. 

---