SECTIONAL TRANSACTIONS.— B. 403 



Afternoon. 

 Visit to the University Departments of Chemistry and Metallurgy. 



Monday, August 22. 



Discussion on Clays (lo.o). 



Chairman : Prof. E. K. Rideal, M.B.E., F.R.S. 



Prof. W. L. Bragg, F.R.S. — General features of the atomic structure 

 of silicates : inferences to be drawn from them as to the structure of 

 clay minerals. 



The minerals found in clay are often of very variable chemical constitu- 

 tion, and are imperfectly crystallised. The evidence as to atomic pattern 

 given by X-ray diffraction is meagre and difficult to interpret. It is there- 

 fore necessary to supplement it by making use of all the knowledge we have 

 about the grouping of atoms in silicates in general, which has been obtained 

 by studying well-crystallised types. 



The silicon-oxygen framework of a silicate, composed of tetrahedral 

 groups linked by their corners, is so rigid and strong that it determines 

 the form of the whole structure. In most, if not all, clay structures, the 

 framework takes the form of sheets of tetrahedra linked by their bases, with 

 free vertices. Such sheets are found in mica, which has been analysed 

 completely. Here they occur in pairs, with vertices opposite each other 

 and linked by aluminium or magnesium atoms, so as to form a strong 

 double sheet. In certain other minerals the sheets are single. 



Such sheets may either be directly superimposed in the mineral, or be 

 separated by intermediate layers containing ions, or water molecules. The 

 physical chemistry of the clays is bound up with the attachment to the 

 sheets, or detachment from the sheets, of ions and molecules. In making 

 hypotheses about the behaviour of clay, one must bear in mind the general 

 principles concerning atomic situations and replacement which have 

 emerged from the study of silicates of all kinds. These will be briefly 

 summarised. 



Dr. G. Nagelschmidt. — Structure and properties of imperfectly 

 crystallised clay minerals (10.20). 



According to their power to diffract X-rays two groups of clay minerals 

 can be distinguished. The minerals of the first group show more perfect 

 crystallisation and give better developed powder diagrams than the minerals 

 of the second group. The first group includes kaolinite and pyrophyllite, 

 and the second group halloysite and montmorillonite. The atomic 

 arrangements in the second group are mainly derived by analogies and 

 require further confirmation. 



Montmorillonite is taken as example of the second, imperfectly crystal- 

 lised group, and its chemical variations are described as isomorphous 

 substitutions within the lattice. These substitutions lead to negative 

 charges, which are compensated by excess cations. The bulk of the excess 

 cations is exchangeable. Montmorillonite shows reversible one-dimen- 

 sional lattice shrinkage and expansion upon variations in water content. 

 The amount of water held in equilibrium depends on the vapour pressure, 

 and, at a given vapour pressure, on the kind of excess cations present. 



