Dehydration–rehydration in magnesium vermiculite: conversion from two–one and one–two water layer hydration states through the formation of interstratified phases

Abstract

The dehydration-rehydration of Mg-vermiculite, as large flakes (7 × 5 × 0.1 mm³) and powder (⩽ 80 µm), has been studied. Samples were treated in a sealed bomb at 140 and 100 °C for several days. After dehydration, they were rehydrated at room temperature. The samples were examined by X-ray diffraction, and the results are analysed here for the first time by the direct Fourier transform method. The results indicated the formation of two-water layer hydration states (14.4, 14.2, and 13.8 Å phases), and one-layer Mg-vermiculite (11.5 Å phase) segregated phases, besides binary interstratified one–two water layer hydration states of Mg-vermiculite phases. The existence of interstratified phases was confirmed in Mg-vermiculite flakes in the conversion from two-one and one–two water layer hydration states in Mg-vermiculite flakes by dehydration-rehydration treatments. The dehydration takes place through interstratified phases consisting of alternations of 13.8 and 11.5 Å components (25.3 Å, regular interstratified phase). The transformation towards the 11.5 Å phase at 140 °C is more rapid than at 100 °C. The rehydration produces mixed layers consisting of alternating 11.5 and 14.2 Å components (random interstratified phases). Using powdered Mg-vermiculite, the changes occur faster than in the flakes, and are produced only at 140 °C. Under these conditions, no interstratification can be detected, and the dehydration-rehydration takes place spontaneously.

The presence of the interstratified phase in the conversion of two–one and one–two water layer hydration states arises from the mobility of the water molecules in the crystals. The results suggest that the large particles permit a low mobility of water molecules and the thermodynamic equilibrium between the original and final segregated phases gives rise to a regular phase, characterized by the loss of an equal number of water molecules in the interlayer space. In the case of rehydration, the process is reversed. The transformation from two- to one-water layer Mg-vermiculite throughout interstratified phases depends on the temperature, time of treatment and particle size.