X-ray and neutron investigations of the P-I transition in anorthite with low albite content

Three different samples with an albite concentration of 2.5% and one with an albite concentration of 4.5% were used to investigate the behaviour of c and d reflections in a temperature range of 143 to 570 K by X-rays and neutrons. As for An₁₀₀ the phase transition P-I is complete and reversible, but in contrast to An₁₀₀ a definite transition point cannot be determined. Furthermore, the temperature range of the phase transition is shifted to lower temperatures. With increasing albite concentration the characteristics of a well-defined phase transition disappear. The temperature dependence of the c and d intensities and the anisotropy of the diffuse scattering contributions are similar to those of An₁₀₀; therefore, they are traced back to the same dynamical origin. The additional influence of antiphase domain boundaries on the line profiles of the c and d reflections is discussed.