Polytype and morphological analyses of gümbelite, a fibrous Mg-rich illite

Abstract

Polytypism in gü mbelite and its relationship to the fibrous or ribbon-like morphology exhibited by this Mg-rich illite were investigated by powder X-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Comparison between the XRD pattern from oriented fibers using a conventional powder diffractometer and a randomly oriented pattern using a Gandolfi camera suggested that 2M 2 is dominant but other polytypes belonging to subfamily A also exist, and that the fiber axis of gü mbelite is parallel to <110> in 2M 2 , <110> in 2M 1 , and <100> in 1M. The EBSD analyses confirmed these crystallographic directions directly from individual crystals. Electron diffraction and high-resolution TEM showed that twinning and intergrowths of various polytypes including both subfamilies are common in a single crystal and that the two types of rotations [2n60º and (2n+1)60º] between adjacent layers are often randomly mixed at the monolayer level. The data suggest that high densities of twinning and intergrowths account for the origin of the fibrous morphology along <110> for 2M 1 and 2M 2 polytypes. Volume restriction in a confined vein space may also play a role.

Visualization of clay minerals at the atomic scale

“…and one of ±Y i (Kameda et al, 2007a). Heavy stacking disorder is observable in the both images, but locally a two-layer polytype (probably 2M 2 ; see Kameda et al (2007)) is formed.…”

“…2). Figure 3 shows HRTEM images of fibrous Mg-rich illite (gümbelite) along one of ± X i and one of ± Y i (Kameda et al , 2007a). Heavy stacking disorder is observable in the both images, but locally a two-layer polytype (probably 2 M 2 ; see Kameda et al (2007)) is formed.…”

“…Noise contrast was removed using a Wiener filter, as described by Kogure et al (2008a), to identify the orientation of each 2 :1 layer. The white bars in (b) are the same as those introduced in Fig. 2 (Kameda et al, 2007a).…”

Structural anomalies in tobelite-2M2 explained by high resolution and analytical electron microscopy

“…The 2O polytype has been reported for anandite (Giuseppetti and Tadini, 1972) and phlogopite (Ferraris et al, 2001). Mixed-rotation polytypes or stacking sequences where both 2n × 60°and (2n + 1) × 60°rotations occur (Nespolo, 1999), have also been reported for annite (Kogure and Nespolo, 1999a), oxybiotite (Kogure and Nespolo, 1999b), lepidolite (Kogure and Bunno, 2004), gümbelite (Kameda et al, 2007) and celadonite (Kogure et al, 2008).…”