Analysis of stacking faults in nacrite by low-dose atomic-resolution imaging using OBF-STEM

The structure of stacking faults in nacrite, a polytype of kaolin group minerals, was revealed by atomic-resolution imaging using aberration-corrected scanning transmission electron microscopy (STEM), particularly by optimum-bright-field (OBF) imaging with a segmented annular detector, which can achieve a sufficient signal-to-noise ratio even at a very low electron dose.The results unambiguously demonstrated that nacrite, which belongs to polytypic Group D of 1:1 phyllosilicates, contains the stacking sequence of Group A as a stacking fault.This result suggests that the stacking sequences of different polytypic groups can coexist in dioctahedral 1:1 phyllosilicates, as well as in trioctahedral ones.Furthermore, the atomic-resolution images of nacrite showed weak contrasts at specific oxygen columns, suggesting partial dehydroxylation in nacrite induced by electron irradiation during imaging or focused-ion-beam (FIB) processing.