The orthoclase-microcline inversion: A high-resolution transmission electron microscope study and strain analysis

High-resolution electron microscopy of an intermediate microcline (Or₉₃) from a granodiorite in southeastern Australia reveals an en echelon arrangement of triclinic lens-shaped domains, twinned on the albite law. The domains are tabular on (010), are only a few unit cells wide, but extend 20 or 30 unit cells along x, until they merge into a zone of monoclinic cells roughly aligned in the rhombic section. The domains are longer and less clearly terminated along z. Strain calculations show that the energy released by Al/Si ordering, producing the orthoclase-microcline inversion, is equal to the strain energy developed when triclinic domains are forced to retain the original monoclinic crystal shape. This balance of strain energies thus explains the metastable persistence of intermediate microcline into the region of maximum microcline stability. Shearing along faults during deformation of the granodiorite released the strain in some of these feldspars, allowing maximum microcline to develop, and so giving rise to a bimodal distribution of triclinicities throughout the pluton. The value of γ measured for the intermediate microcline is the average of a range of values throughout each domain, and may be considerably closer to 90° than γ from an unstrained crystal with the same degree of Al/Si order.