Twin memory and twin amnesia in anorthoclase

Many natural minerals and synthetic materials display twin microstructures resulting from displacive phase transitions. These microstructures may be r emoved temporarily from the sample by heating above the relevant transition temperature, though the twinning generally returns on subsequent cooling. In anorthoclase, the spatial distributions of twins before and after brief annealing above Te are often identical. This propel ty appears to be a comm on feature in many materials which undergo ferroelastic phase transitions, anti is known as 'twin memory'. The atomic mechanisms responsible for this twin memory may be investigated by studying the annealing regimes required to remove the memory effect, how lung must a sample be annealed. and at wha t temperature, to induce 'twin amnesia'. High-resolution X-ray diffraction (XRD) has been used to investigate twin m emory and twin amnesia in anorthoclase. In anorthoclase, the primary constr aint on twin amnesia is thermodynamic, rather than kinetic. The critical te mperature to induce amnesia correlates well with the top of the (Na, K) sol vus in disordered alkali feldspar. For this reason, the proposed mechanism for twin memory involves the segregation of alkali cations in thin lamellae at the twin boundaries.