MS-13-P-3259 Electron Microscopy of Smectite Clay and the Assembly

Smectite, the major clay minerals in bentonite, is one of the most interesting and ubiquitous clay minerals in the mineral kingdom. It holds a special place in scientific research because of its great impact on human daily life, e.g., process industries, civil engineering and construction, geology and petroleum, agriculture and food, medicine and pharmaceutical industries, materials science and engineering. The precise characterization of smectitic clays, especially particle size and shape, is crucial in clay research, but is challenging due to particle aggregation (i.e., large surface area to volume ratios and high chemical activity on the clay surface). This paper introduces novel methods of studying smectitic clay particles and their assembly using electron microscopy.

Smectite clay usually occurs only in very small particles. Bentonite clays collected from different localities were purified and dispersed in de-ionized water without any dispersion agent (to avoid possible artifacts). Particle size of < 0.2 μm fraction was collected by the settling (pipet) method. These clay suspensions were very carefully transferred into a special wet environmental cell and then inserted into the TEM column for in situ (WETEM) study [1, 2]. The traditional air-dried, cryo-TEM and SEM methods were also carried out for comparison purpose. Conventional embedding and ultra-microtoming techniques and FIB method were used for clay assembly research.

Conventional electron micrographs of smectite showed broad undulating mosaic sheets, irregular masses of extremely small particles, and irregular flake-shaped aggregates (Fig. 1), though elongated lath-shaped units with flake-, needle-, and rod-like particles were also found. Usually, individual particles can barely be discerned due to particle aggregation in vacuum environment, but the use of WETEM has led to the discovery of different shapes (granular or spherical-like, elongated fiber-like, needle, triangular, polygon and others) of nano-size smectitic clays in addition to those previously reported (Fig. 2). Particle size analysis of ~20 smectite samples showed that almost all smectite particles ranged from 5 to 700 nm in equivalent diameter with a mode between 30 to 125 nm and a mean between 90 to 300 nm regardless of the clay fraction obtained by settling method. Clay aggregates observed in both conventional TEM, WETEM, and SEM revealed many interesting features. With advanced hard and soft wares, surface 3-D images and 3-D tomography can be constructed by images obtained from SEM, TEM and FIB (Fig. 3). The application of 3-D analysis of clays and its assembly promises to increase our understanding and improve the application of clays in many aspects.