PM-08CBED analysis of iridescent garnets from Tenkawa, Nara Prefecture, Japan

Abstract

Grossular (Ca 3 Al 2 Si 3 O 12 ) - andradite (Ca 3 Fe 3+2 Si 3 O 12 ) garnet solid solutions (space group, Ia-3d), termed grandite series, often exhibit optical birefringence and iridescence. Iridescent garnet from Tenkawa, Nara Prefecture, Japan, has the fine lamellar texture (ca. 100-300 nm in thickness) with small differences in Al/Fe 3+ ratio by transmission electron microscope (TEM) (e.g., Shimobayashi et al., 2005). Our previous studies (Chang et al., 2016, 2017) reported that extra reflections, 110, 200, 411 and so on, to break the symmetry of Ia-3d were detected in selected area electron diffraction (SAD) patterns from Al-rich fine lamellae and the crystal family of this Al-rich fine lamella is triclinic and lattice type is I-lattice and suggest the space group is I-1 or I1 in the convergent-beam electron diffraction (CBED) pattenrs. Chang et al. (2017) also reported that CBED patterns in the core of lamella and around the boundary between Al-rich and Fe-rich lamellae were different. In this study, we examine CBED by scanning TEM