Crystal Growth of Smectite: A Study Based on the Change in Crystal Chemistry and Morphology of Saponites with Synthesis Time

Smectites are 2:1 type swelling clay minerals widely distributed in the earths crust. The unique structures and properties endow them with various applications in many fields. However, their growth process and mechanism are still not completely understood. In this study, we tried to understand this growth process and pattern from the perspective of crystal chemistry and morphology. Smectites were synthesized by the hydrothermal treatment of starting materials at 220 degrees C under autogenous pressure from 0.25 to 30 days. The starting materials were obtained from the mixtures of Mg- and Ni-smectite precursors synthesized at 150 degrees C for 1 day with identical molar weights. The starting materials, as well as further synthesized samples, are identified as smectites, saponite-type, by X-ray diffraction analysis and Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of the starting materials exhibited the nu Mg3OH and nu Ni3OH bands only. After the hydrothermal treatment (220 degrees C) of the starting materials, the nu Mg2NiOH and nu Ni2MgOH bands appear and relatively increase with the prolonged treatment, indicating the appearance of neighboring Mg-Ni in the octahedral sheet of saponite and thus the crystallization of the newly formed saponite with a mixed composition. Argon adsorption and scanning transmission electron microscopy (STEM) results are consistent and indicate that the mean particle size of the synthesized saponites increases for a short duration and tends to remain stagnant from 14 to 30 days. The evolution of Mg and Ni distribution and of the particle size from argon adsorption and STEM imply the following: (i) the distribution of the octahedral cations tends to be random; (ii) the combination of argon adsorption and STEM offers a powerful evidence for the increase of particle size; and (iii) crystal growth proceeds via a lateral extension of the layers by surface-controlled growth. The experimental results indicate the viability and effectiveness of coupling FTIR, argon adsorption, and STEM in illuminating the crystal chemistry and morphology at the early stage of the crystal growth process of smectite.