Clay Under High H2O Pressures

A piece of Ca-montmorillonite speciment with H2O in a DAC sample chamber

Introduction

Montmorillonite is an aluminous dioctahedral 2:1 layer (2 tetrahedral layers : 1 octahedral layer) clay mineral (Bailey, 1980), belonging to the smectite group. The interlayer cations in montmorillonite are very easily hydrated. It is well established that the different amounts of water are absorbed at each stage, observable in the hydrothermal diamond-anvil cell by in-situ X-ray diffraction at CHESS. The dehydration-rehydration cycle can be easily observed on a temperature vs. d(001) plot for various isochore

At pressures between the H2O liquid-vapor (L-V) boundary and ~10 kbar, the dehydration temperature for the conversion from the 19-A hydration state to the 15-A hydration state occurred over the range 260-350 C for Ca-montmorillonite and 200-250 C for Mg-montmorillonite, with a slight increase with increasing pressure. For both materials, the rehydration from 15-A to 19-A state occurred at the same temperature as dehydration at pressures along the H2O L-V boundary, thus showing no hysteresis. The rehydration hysteresis increased to nearly 75 C at 6 kbar for Ca-montmorillonite and to the same amount at 2.5 kbar for Mg-montmorillonite.

Structures of aluminous dioctahedral 2:1 layer sheet silicate P-T paths along H2O isochores Series of in-situ X-ray diffraction patterns d(001) vs. temperature for dehydration-rehydration cycle between 19-A and 15-A hydration state of Ca-montmorillonite P-T diagram showing the dehydratiton and rehydration temperature of Ca-montmorillonite in Word 6.0(Mac), or in RTF format