Effect of Pressure Treatment on the Specific Surface Area in Kaolin Group Minerals

Soyeon Kwon,Yongjae Lee,Huijeong Hwang

doi:10.3390/cryst9100528

Abstract

Pressure can tailor the properties of a material by changing its atomistic arrangements and/or crystal morphology. We have investigated the changes in the adsorption properties of kaolin group minerals as a function of pressure treatment in the gigapascal range. External pressures have been applied using a large volume press (LVP) to kaolinite (Al2Si2O5(OH)4) and halloysite (H4Al2O9Si2·2H2O), which represent natural 2D layered and 1D nanowire structures, respectively. Powdered samples have been compressed up to 3 GPa in 1 GPa steps at room temperature and recovered by up to ca. 0.35 g from each pressure step. Brunauer–Emmett–Teller (BET) measurements were conducted using N2 gas to measure the specific surface area, pore size distribution, and pore volume of the pressure-treated samples. As the treatment pressure increased, kaolinite showed an increase in the adsorption behavior from nonpores to mesopores, whereas halloysite responded in an opposite manner to show a decrease in its adsorption capability. We discuss the contrasting effects of pressure-treatment on the two morphologically distinct kaolin group minerals based on field-emission scanning electron microscope (FE-SEM) images measured on each recovered material. We observed that the layers in kaolinite separate into smaller units upon increasing pressure treatment, whereas the tubes in halloysite become flattened, which led to the contrasting changes in surface area. Further study is in progress to compare this effect to when water is used as the pressure-transmitting medium.

Highlights

Kaolinite and halloysite are representative clay minerals belonging to the kaolin group and are layered silicate minerals making up fine particles [1]
The kaolinite used in this study was a KGa-1b sample approved by Clay Minerals Society
Powder diffraction patterns indicate that the long-range ordering of kaolinite and halloysite is maintained after pressure treatment without showing no apparent changes in peak positions and shape (Figure 1)

Summary

Introduction

Kaolinite and halloysite are representative clay minerals belonging to the kaolin group and are layered silicate minerals making up fine particles [1]. A recent study on kaolinite has shown that under high-temperature and high-pressure conditions mimicking a depth of ~75 km along a cold subduction zone, water molecules can be inserted into kaolinite layers, making super-hydrated kaolinite, Al2 Si2 O5 (OH)4 ·3H2 O, with a water content higher. Crystals 2019, 9, 528 inserted into kaolinite layers, making super-hydrated kaolinite, Al2Si2O5(OH)4·3H2O, with a water content higher than that of halloysite [11]. The geochemical seismic implications from the formation fromthat theof formation super-hydrated changesand in physical properties are expected, which of super-hydrated kaolinite, changes in physical properties are expected, which might be selectively might be selectively utilized in certain applications using these type of clay minerals.

Materials and Methods

High-Pressure Treatment

Results

Conclusions