Calculations on a model intercalate containing a single layer of water molecules: a study of potassium vermiculite, K 2x Mg 6 (Si 4 _ x Al x ) 2 O 20 (OH) 4 .(H 2 O) 4 , for 1 ≤ x ≤ 0

Abstract

The largest body of information concerning the swelling of clay minerals on exposure to moisture has been accumulated for the vermiculites and the smectites. The experimental findings for a range of phyllosilicates possessing various interlayer charges have shown that for silicates with zero charge on the interlayer (for example talc or pyrophyllite) there is no intercalation of water molecules within the sheets. Maximum water uptake, in this class of minerals, is found to take place in those having a fairly low interlayer charge (for example hectorites, montmorillonite). As the layer charge is increased further uptake falls to zero for fully saturated interlayers (for example beidellite and vermiculites). This paper represents a comprehensive study from a theoretical standpoint of some of these observations and enables comment to be made on the influence of layer charge on the intercalation process in addition to the examination of other features that have been held to be important (for example tetrahedral or octahedral site substitution) so enabling discussion of the work of the experimenters in this area. Calculations are reported for the electrostatic energy of a potassium vermiculite, K 2x Mg 6 (Si 4 _ x Al x ) 2 O 20 (OH) 4 .(H 2 O) 4 , which contains a single layer of water molecules intercalated into an expanded phlogopite. The crystal structure adopted is such that for x = 1, K 2 Mg 6 (Si 3 _ x Al) 2 O 20 (OH) 4 .(H 2 O) 4 , the oxygen atoms of the water molecules form a honeycomb-like network with the potassium ions in the holes. For x = 1, the calculation is straightforward and is based on our ‘generic’ approach reported previously. This approach also enabled us, in one single calculation, to study the influence of interlayer K + ions placed at various levels between the water layer and the silicate layer. It appears that the most favourable position depends on the charge distribution within the water molecule. The centrally positioned K + ion represents the most stable configuration when the charge on the hydrogen atom of the water q H " ≥ 0.6. For q H " below this value the most stable position for the K+ ions is between the water layer and the plane of the nearest oxygens in the silicate sheet. For x < 1 specific considerations had to be given to the likely arrangement of interlayer K + ions and water molecules (taken to be in the plane of the water layer), implying separate calculation of their interaction energies. The intercalation energy comprises the energy involved in the process of expanding a phlogopite from 10 A + to 12.5 A and arranging water molecules in the gap. It emerges that the intercalation is mainly determined by the expansion energy. Both the expansion energy and the arrangement energy depend on the silicate layer charge, which is determined by x. For large x no intercalation occurs owing to the large amount of energy required for expansion. For x very close to zero no intercalation occurs owing to the repulsive interaction of the water layer with the silicate layer. For intermediate values of x intercalation occurs owing to the large K -H 2 O interaction energy.