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TEM investigation of Ca-rich plagioclase: Structural fluctuations related to the I-P phase transition

¹ School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, U.S.A.
² Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, U.S.A.
³ Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, Parma, 43100, Italy
⁴ Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, Torino, 10125, Italy

Correspondence: ^* E-mail: pbuseck{at}asu.edu

We used transmission electron microscopy (TEM) to understand microstructural changes accompanying the reversible I-P phase transition in Ca-rich plagioclase as a function of Na content. The transition produces c- and d-type domains that have P symmetries, and the symmetries of domain walls are I. 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 Al-Si framework and single Ca sites for the I domains. The P domains of anorthite with high Al-Si ordering are homogeneous. In contrast, structural fluctuations occur in both anorthite with lower Al-Si ordering and in Na-bearing plagioclase. The magnitudes of the fluctuations decrease with increases in Na content.

Key Words: I-P phase transition • Ca-rich plagioclase • HRTEM • structural fluctuations