A study of optimal phase boundaries: the case of exsolved alkali feldspars

A general theory of grain and phase boundaries (the O-lattice theory) is further developed and tested on alkali feldspars with exsolution lamellae, since measurements exist for the structures of the two-phase system (perthitic feldspars) as well as for the orientation of the phase boundary. It is shown that in this case the adaptation of two monoclinic structures is energetically preferable to the adaptation of a monoclinic and a triclinic one. The phase boundary energy is markedly lower in the former case. Thus, a pseudo-monoclinic structure is produced out of the triclinic by periodic submicroscopic twinning. The calculated orientation of the phase boundary is in close agreement with the measurements.