Local low-symmetry structure in iridescent garnet detected via spatially resolved electron diffractometry
Year: 2025
Type: article
Abstract: The crystallographic structure of a low-symmetry phase locally present in an iridescent garnet grain from Tenkawa Village, Nara Prefecture, Japan, was identified via spatially resolved electron diffractometry, specifically four-dimensional scanning transmission electron microscopy (4D-STEM). Investigations revealed that the lamellae texture parallel to the growth plane is composed of an Al-poor andradite region with an I41/a3¯2/d (full notation for Ia3¯d) cubic structure, and an Al-rich region with lower symmetry. The lower symmetry structure is approximately characterized by octahedral Al-Fe ordering following the I13¯2/d (unconventional setting for R3¯c) symmetry, although the actual symmetry may be even lower, I112/d (unconventional setting for C2/c). The region containing the low-symmetry structure has pseudo-merohedral twinning textures, causing slight variations in the appearance of extra reflections in the diffraction pattern in the nanometer order. These results imply that the I13¯2/d-type octahedral Al-Fe ordering has relatively high thermodynamic stability at low temperatures for compositions around Adr80Grs20. The implemented spatially resolved diffractometry method is an efficient way to further advance the mineralogy of low-symmetry garnets and other unrevealed phase relationships.
Source: American Mineralogist
DOI: 10.2138/am-2025-9870
Authors: Yohei Igami, Akira Miyake, Yuhchyuan Chang, Norimasa Shimobayashi
Institution: Kyoto University
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W7115684768