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

The morphological characteristics of the Zhongxiang （钟祥） 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）4^＊nH2O. 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.