Abstract
High-pressure synchrotron X-ray powder diffraction studies of smectite group minerals (beidellite, montmorillonite, and nontronite) reveal comparative volumetric changes in the presence of different fluids, as pressure transmitting media (PTM) of silicone oil and distilled water for anhydrous and hydrous environments at room temperature. Using silicone oil PTM, all minerals show gradual contraction of unit-cell volumes and atomistic interplane distances. They, however, show abrupt collapse near 1.0 GPa under distilled water conditions due to hydrostatic to quasi-hydrostatic environmental changes of water PTM around samples concomitant with the transition from liquid to ICE-VI and ICE-VII. The degrees of volume contractions of beidellite, montmorillonite, and nontronite up to ca. 3 GPa are ca. 6.6%, 8.9%, and 7.5% with bulk moduli of ca. 38(1) GPa, 31(2) GPa, and 26(1) GPa under silicone oil pressure, whereas 13(1) GPa, 13(2) GPa, and 17(2) GPa, and 17(1) GPa, 20(1) GPa, and 21(1) GPa under hydrostatic and quasi-hydrostatic environments before and after 1.50 GPa, respectively.
Highlights
Smectite is the most abundant two-dimensional phyllosilicate mineral group and comprises the most common natural materials, which occur by weathering, diagenesis, and hydrothermal alteration [1,2]
Pressure-dependent changes in the measured in-situ synchrotron X-ray powder diffraction patterns of smectites under anhydrous and hydrous conditions are shown in Figures 2 and 3 and Tables S1–S3
Smectites in a water-saturated environment showed noticeable extension of d(001) up to ca. 19 Å, which was interpreted as interlayer expansion due to formation of tri-water layers, whereas d(001) of all samples in a silicone oil environment was constant at ca. 15 Å, which is similar length of bi-water layers (Figure 4c,d) [20]
Summary
Smectite is the most abundant two-dimensional phyllosilicate mineral group and comprises the most common natural materials, which occur by weathering, diagenesis, and hydrothermal alteration [1,2]. It is an important material for applications such as engineered barrier systems for nuclear waste storage and industrial catalysts [3,4]. Representative minerals in the smectite group are beidellite, montmorillonite, and nontronite stratified by 2:1 (T–O–T) layered framework and counterbalance cation surrounded by water molecules at expandable interlayers [5,6,7]. A synthetic Na–hectorite (Na0.3 (Mg) (Si4 O10 )(F)2 -xH2 O), Materials 2020, 13, 3784; doi:10.3390/ma13173784 www.mdpi.com/journal/materials
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