Morphological Characteristics of (K, Na)-Rectorite from Zhongxiang Rectorite Deposit, Hubei, Central China

Abstract

The morphological characteristics of the Zhongxiang () rectorite have been studied using X-ray diffraction (XRD), electron probe micro-analyzer (EPMA), scanning electron microscopy (SEM), atomic force microscope (AFM), and high-resolution transmission electron microscopy (HRTEM). The structural formula of the Zhongxiang rectorite is: (Na0.45K0.32Ca0.37Mg0.08)(Al3.78Fe0.11Ti0.10)[(Si6.22Al1.78)O20](OH)4nH2O. It should be confined to a regular interstratification of (K, Na)-mica and Ca-montmorillonite. SEM observations show that Zhongxiang rectorite occurs as platy and fold-shaped crystals, and mainly as extremely thin plates with thickness ranging from 0.4 to 0.05 μm and a smooth (001) surface. There are well-developed polygonal steps on the surfaces of some thick crystals, suggesting a layer-by-layer growth mechanism. AFM observations show a series of steps with a height of 2 nm on the platy particles, suggesting the stacking of 20 nm fundamental particles. Club-like or fiber-shaped halloysite is included in the platy crystals with their elongated dimension paralleling (001) of the platy crystals or crossing the (001) surface of the platy rectorite, indicating multi-stage crystallization and involvement of hydrothermal fluids. The Zhongxiang rectorite was generated by both layer-by-layer growth mechanism and dissolution and crystallization growth mechanism with multistages.