POLYTYPE AND MORPHOLOGICAL ANALYSES OF GÜMBELITE A FIBROUS MG-RICH ILLITE

Abstract

Polytypism in gümbelite 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 2M2 is dominant but other polytypes belonging to subfamily A also exist, and that the fiber axis of gümbelite is parallel to in 2M2, in 2M1, and 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 [2n60° and (2n+1)60°] 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 for 2M1 and 2M2 polytypes. Volume restriction in a confined vein space may also play a role.