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 removed 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 T (sub C) are often identical. This property appears to be a common feature in many materials which undergo ferroelastic phase transitions, and 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 long must a sample be annealed, and at what temperature, to induce "twin amnesia". High-resolution X-ray diffraction (XRD) has been used to investigate twin memory and twin amnesia in anorthoclase. In anorthoclase, the primary constraint on twin amnesia is thermodynamic, rather than kinetic. The critical temperature to induce amnesia correlates well with the top of the (Na, K) solvus 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.