THICKNESS OF PERICLINE TWIN WALLS IN ANORTHOCLASE - AN X-RAY-DIFFRACTION STUDY

The thickness of pericline twin walls in disordered feldspar Ab(65)Or( 30)An(5) has been determined by quantitative measurements of the diffu se X-ray diffraction caused by these walls. The diffuse diffraction ap pears as streaking between pairs of twin-related Bragg peaks. Comparis on of the predictions of simple model calculations with the observed i ntensity profiles indicates that these walls are approximate to 25 Ang strom thick at room temperature. On heating, the wall thickness increa ses, following the predictions of Ginzburg-Landau theory. W proportion al to \T-c - T\(-1/2).