High pressure behavior of layered hydrous silicate, kaolinite: Insights from Raman spectroscopy

Abstract:

Kaolinite (Al₂Si₂O₅(OH)₄) is an important mineral phase abundant in the oceanic sediments and helps in transporting water into the deep Earth via subduction zones. Recent X-ray diffraction studies have revealed pressure induced hydration in a natural sample of kaolinite at subduction zone conditions. This is a novel finding contrary to expectations. Instead of contracting significantly under compression, kaolinite was found to expand along the interlayers undergoing intercalation. To examine the anomalous high-pressure intercalation in kaolinite, we have undertaken Raman scattering measurements in diamond anvil cell up to ~10 GPa. We document the effect of pressure on the weakly hydrogen-bonded interlayer region by observing the pressure dependence of the hydroxyl stretching modes. We also examine the low wavenumber lattice region. Our preliminary findings indicate that, up to pressures of around 2.7- 3.0 GPa, there is a red-shift of the hydrogen stretching modes. And beyond 3.0 GPa, there is a discontinuous increase in the hydroxyl stretching frequency followed by a blue shift of the hydrogen stretching modes.

The low pressure red shift implies that the hydrogen bonding is strengthened in the interlayer region. In contrast, the discontinuity and the following blue shift possibly alludes to a snapping of hydrogen bonding. We conclude that at ~ 3.0 GPa, kaolinite structure is devoid of hydrogen bonding and primed for intercalation of water molecules.

Acknowledgment: AB acknowledges the Dean’s post-doctoral fellowship from Florida State University. This work is supported by funds from NSF EAR1638752 and EAR1753125.