Electron microscopic study of the structural changes as a function of temperature in anorthite*

Abstract

Abstract Diffraction patterns, bright-field images and dark-field images, mainly with so called c reflections (h + k + l = odd), were taken from anorthite (CaAl2Si2O8) crystals at different temperatures. From the results follows: At temperatures somewhat above room temperature large antiphase domains exist with a displacement vector R = ½[111]. The anti-phase domains are associated with the whole structure. The size of the anti-phase domains does not change very much with increasing temperature although the anti-phase boundaries seem to be somewhat mobile. At the critical temperature Tc = 230°C the anti-phase domains disappear. Above Tc dark-field images with c reflections show bright patches in a dark matrix. The bright patches are probably caused by the Ca atoms only which, in contrast to the body-centred (Si,Al)O2 framework, show a primitive ordered arrangement in some elementary cells. In the dark parts of the images the Ca atoms are either body centred or disordered.