High-pressure Raman scattering and X-ray diffraction study of kaolinite, Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>

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.

高岭石由大陆地壳岩石风化形成，在热带地区的海洋沉积物中也有发现。高岭石和其他层状含水硅酸盐矿物可能在通过俯冲板块将水输送到地球内部的过程中起着至关重要的作用。近期研究通过实验记录了在高压下（即压力诱导水合作用）水分子嵌入导致的层间区域扩张。这是违反直觉的，因为层状硅酸盐中的层间区域是相当可压缩的，所以理解导致层间区域嵌入和扩张的潜在机制是很重要的。 为了解决这个问题，我们研究了来自爱荷华州基奥卡克的天然高岭石在高压下的行为。该样品不含锐钛矿杂质，因此有助于利用拉曼光谱和基于同步加速器的粉末X射线衍射来检测低能（0 - 1200 cm⁻¹）和高能羟基（3000 - 4000 cm⁻¹）区域。 我们的结果表明，羟基模式的压力依赖性在约3 GPa和约6.5 GPa处呈现不连续性。这与高岭石从K - 1相到K - II相以及从K - II相到K - III相的多型转变有关。几个低能拉曼模式的压力依赖性也表现出类似的不连续行为。基于同步加速器的粉末X射线衍射结果也表明晶胞体积和晶格参数的压力依赖性存在不连续行为。对体积和线性压缩率的分析表明，高岭石具有极强的各向异性，这可能有助于其在俯冲带环境中的地球物理可探测性。从K - I到K - II的多型转变以氢键的断裂为标志，因此在与地球内部相关的条件下，水分子嵌入层间区域，稳定晶体结构，并有助于形成超水合高岭石，这种高岭石可以将更多的水输送到地球内部。