Local low-symmetry structure in iridescent garnet detected via spatially resolved electron diffractometry

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.