LOW-DOSE ADF-STEM STUDY OF SERPENTINE

Layer silicates are secondary clay minerals that exhibit extensive defects including polytypism, i.e., alternative layer stacking arrangements, and interstratification, i.e., incorporation of compositionally distinct mineral phases. Fundamental models for layer silicate stacking and interstratification have been derived from X-ray and electron diffraction methods and from high-resolution transmission electron microscopy. Clay minerals are typically compositionally diverse due to the presence of metal substitutions, however, and the relationships between composition and defect structure remain incompletely understood. Direct observation and identification of atomic columns in clay minerals would greatly aid the development of models for stacking defects but has been extremely challenging due to the sensitivity of clay minerals to electron beam damage. Here we present new observations of the structure of a chromium-rich, magnesium-substituted aluminosilicate, amesite, using low-dose annular dark-field transmission electron microscopy. Using an electron dose as low as ~6000 electrons/Ų, we clearly observed all the cations and O atomic columns for the first time. It was found that the amesite from Saranovskii mine belongs to the group D serpentine but has a random combination of -1/3b, 0b, 1/3b shifts. The relative abundance of 6R₂, 6R₁, 2H₁ and 2H₂ polytype is 46.1%, 29.6%, 7.7% and 1.9% respectively. The new imaging technique also enables us to unravel the structural and chemical features of serpentine-chlorite stacking defects, where the chlorite sheet is determined to be IIb type and Cr slightly enriches at the octahedral layer of the ‘talc’ sheet. The results presented here could not only open a new venue for the clay mineralogy but also be leveraged to the structural characterization of other beam-sensitive materials like zeolite and metal-organic framework.