TWINNING IN SYNTHETIC ANORTHITE - A TRANSMISSION ELECTRON-MICROSCOPY INVESTIGATION

Transmission electron microscopy (TEM) has revealed dense, complex twi ns in synthetic anorthite crystals formed by annealing CaAl2Si2O8 glas s. The crystals are dominated by Carlsbad and Carlsbad-albite twins, w ith small amounts of albite twins. All the composition planes are para llel to (010). High-resolution TEM images show the twin boundaries to be coherent. Selected-area electron diffraction (SAED) patterns contai ning the b-c* reciprocal plane are useful for distinguishing these tw ins. Extremely dense Carlsbad twins at the unit-cell scale indicate th at additional energy caused by the Carlsbad twin boundaries in highly disordered anorthite is lower than that caused by albite twin boundari es. We propose that for anorthite with an initially disordered structu re the total energy induced by Carlsbad twinning is lower than that ca used by albite twinning and higher than that caused by albite twinning in anorthite with an initially ordered structure.