High-pressure Raman scattering and X-ray diffraction study of kaolinite, Al2Si2O5(OH)4

Abstract

Kaolinite is formed by weathering of continental crustal rocks and is also found in marine sediments in the tropical region. Kaolinite and other layered hydrous silicate minerals are likely to play a vital role in transporting water into the Earth's interior via subducting slabs. Recent studies have experimentally documented the expansion of the interlayer region by intercalation of water molecules at high pressures i.e., pressure-induced hydration. This is counter-intuitive since the interlayer region in the layered silicates is quite compressible, so it is important to understand the underlying mechanism that causes intercalation and expansion of the interlayer region.To address this, we explore the high-pressure behavior of natural kaolinite from Keokuk, Iowa. This sample is free of anatase impurities and thus helps to examine both low-energy (0–1200 cm−1) and high-energy hydroxyl (3000–4000 cm−1) regions using Raman spectroscopy and synchrotron-based powder X-ray diffraction.Our results show that the pressure dependence of the hydroxyl modes exhibits discontinuities at ∼3 GPa and ∼ 6.5 GPa. This is related to the polytypic transformation of Kaolinite from K-1 to K-II and K-II to K-III phase. Several low-energy Raman modes' pressure dependence also exhibits similar discontinuous behavior. The synchrotron-based powder X-ray diffraction results also indicate discontinuous behavior in the pressure dependence of the unit-cell volume and lattice parameters. The analysis of the bulk and the linear compressibility reveals that kaolinite is extremely anisotropic and is likely to aid its geophysical detectability in subduction zone settings. The K-I to K-II polytypic transition is marked by the snapping of hydrogen bonds, thus at conditions relevant to the Earth's interior, water molecules intercalate in the interlayer region and stabilize the crystal structure and help form the super-hydrated kaolinite which can transport significantly more water into the Earth's interior.