Dynamic character of the primitive to body-centered phase transition in anorthite

We show from elastic neutron diffraction data that anorthite CaAl₂Si₂O₈ (An₁₀₀) undergoes a primitive to body-centered phase transition at T _c =237±1°C. The transition is reversible, and T _c is well defined. Our measurements demonstrate that the I-lattice at the high temperature phase applies to all structural elements; in other words, the time-averaged lattice is exactly body-centered and not just in the space average of An₁₀₀, as concluded earlier by other authors.