TEM investigation of Ca-rich plagioclase: Structural fluctuations related to the I1̄-P1̄ phase transition

We used transmission electron microscopy (TEM) to understand microstructural changes accompanying the reversible I1̄-P1̄ phase transition in Ca-rich plagioclase as a function of Na content. The transition produces c- and d-type domains that have P1̄ symmetries, and the symmetries of domain walls are I1̄. Based on diffractograms and contrast differences of high-resolution TEM (HRTEM) images, we infer that structural differences occur on a scale of tens of nanometers, giving rise to structural fluctuations. These variations can be followed on a nanometer scale using HRTEM images. We propose a model with a body-centered A1-Si framework and single Ca sites for the I1̄ domains. The P1̄ domains of anorthite with high A1-Si ordering are homogeneous. In contrast, structural fluctuations occur in both anorthite with lower A1-Si ordering and in Na-bearing plagioclase. The magnitudes of the fluctuations decrease with increases in Na content.