Polytypism in semi-disordered lizardite and amesite by low-dose HAADF- STEM

Abstract

Serpentine minerals exert important controls on the physical properties of ultramafic rocks and have the potential to influence deformation phenomena in fault zones and to control the release of water in subducted slabs. Sheet serpentine generally, and lizardite and amesite specifically, can adopt alternative crystallographic stacking arrangements called polytypes. Polytypism has

Atomic Perspective on the Serpentine–Chlorite Solid-State Transformation

“…The inheritance of octahedral tilts also occurs in the chloritization of long-period lizardite structures, which are significantly less common than the two-layer lizardite structure, 33,36,40 which is based on alternating octahedral tilts, as shown in Figure 1a. Figure 4a shows the co-existence of three-layer (Figure 4b), partially (Figure 4c), and completely (Figure 4d) chloritized lizardite layers.…”

“…We studied a sample of Cr-containing lizardite (Cr-lizardite) that is intergrown with chlorite from Woods Chrome Mine in the State Line Serpentinite, Lower Britain Township, Lancaster County, Pennsylvania . The sample was from the same specimen used in a prior study ref . Both lizardite and chlorite were observed in the sample.…”

“…Recently, we achieved 1.0 Å resolution in HAADF-STEM images of serpentine polytypes with doses of ~6000 e -/Å 2 . 31 Here, we report the results of a high-resolution study that clearly resolved the structural and chemical details of the transformation pathway that leads to chlorite in the hydrogen-bonded configuration. We provide a model for the transformation mechanism that is applicable to all lizardite precursor polytypes that is based on the direct observation that the tilts of the cationoxygen octahedra are always preserved throughout the transformation.…”

“…The overall configuration is IIab, 43 which accounts for 86% in the multiple chlorite layers with the balance being 14% IIbb. The inheritance of octahedral tilts also occurs in the chloritization of long-period lizardite structures, which are significantly less common than the 2-layer lizardite structure 31,34,40 , which is based on alternating octahedral tilts, as shown in Fig. 1a.Fig.…”

“…Serpentine minerals are less beam-sensitive than halloysite and montmorillonite, so they are amenable to low-dose HAADF–STEM, which forms images using only a small fraction of the electrons that are scattered to high angles. Recently, we achieved 1.0 Å resolution in HAADF–STEM images of serpentine polytypes with doses of ∼6000 e – /Å 2 …”