Polytype and polymorph identification of finely divided aluminous dioctahedral mica individual crystals with SAED. Kinematical and dynamical electron diffraction

Abstract

International audienceThis work investigates the potential of selected-area electron diffraction (SAED) for the polytype and polymorph identification of finely divided K-bearing aluminous dioctahedral mica. Individual mica crystals may indeed differ by their layer-stacking sequence and by the inner structure of their octahedral sheets (polytypic and polymorphic variants, respectively). This diversity of natural mica is commonly considered to be responsible for their morphological variety. The present article thus analyzes the intensity distribution between hk0 beams as a function of the crystal structure and thickness. The comparison of ED calculations with experimental diffraction data shows that predicted dynamical effects are not observed for finely divided dioctahedral mica. The influence of different structure defects on calculated intensities is analyzed, and their widespread occurrence in natural mica is hypothesized to be responsible for the limitation of dynamical diffraction effects. SAED may thus be used to identify the structure of individual dioctahedral mica crystals using the kinematical approximation to simulate and qualitatively interpret the observed intensities

Polymorph and Polytype Identification from Individual Mica Particles Using Selected Area Electron Diffraction

“…Dynamical effects are minor for most fined-grained mica crystals of thickness 10-25 nm if the structure consists essentially of light elements (Gaillot et al 2011). In addition, natural micas occur as mosaic crystals, thus reducing the likelihood of multiple scattering and further minimizing dynamical effects on ED patterns (Drits 1987;Gaillot et al 2011).…”

“…Dynamical effects are minor for most fined-grained mica crystals of thickness 10-25 nm if the structure consists essentially of light elements (Gaillot et al 2011). In addition, natural micas occur as mosaic crystals, thus reducing the likelihood of multiple scattering and further minimizing dynamical effects on ED patterns (Drits 1987;Gaillot et al 2011). To assess this assumption, SAED patterns were collected using 1° and 2° precession angles of the incident electron beam to minimize dynamical diffraction effects (Vincent & Midgley 1994), and compared with ED patterns obtained with no beam precession (Fig.…”

“…Periodicity normal to 2:1 layers and occurrence and nature of stacking faults can be determined from electron diffraction (ED) patterns or high-resolution images collected with the incident electron beam parallel to the layers (Kogure & Banfield 1998;Kogure & Nespolo 1999;Kameda et al 2007;. As a possible alternative to the latter approach, Gaillot et al (2011) showed that ED patterns collected along the [001] zone axis can also be used to determine the periodicity along the stacking direction from hk0 intensities and to obtain structural information such as location of the octahedral vacancy. This method is more limited than when crystals are oriented normal to [001].…”

“…In contrast, conventional parallel-illumination ED (selected-area electron diffraction, SAED) requires very low electron dose compared to CBED, while producing averaged intensities over a larger ZOLZ area (Vincent & Midgley 1994). SAED allows retrieving layer stacking information in the direction parallel to the electron beam (Gaillot et al 2011).…”

“…However, intrinsic properties of mica crystals further impair quantitative analysis. The coexistence and interstratification of polytypic and polymorphic varieties within a crystal was addressed by Gaillot et al (2011), together with the impact of variable crystal thickness on mica ED patterns. In addition, mica plates may be bent, or have growth steps, or variable chemical compositions, which result in inhomogeneous local diffraction conditions.…”