404 SECTIONAL TRANSACTIONS.— B. 



The study of the structure and properties of these minerals is important 

 for the understanding and control of many processes in the ceramic and 

 bleaching industries, and in soil management. 



Discussion (10.40). 



Dr. R. K. ScHOFiELD. — Origin of the electric charges on clay particles 

 (ii.io). 



Clay particles are generally electrically charged and therefore retain an 

 equivalent quantity of ions which can only be removed by exchange with 

 other ions carrying the same charge. Exchange of ions is of great import- 

 ance in the industrial handling of clay and in land reclamation. 



Some of the charge on clay particles is due to isomorphous substitutions 

 within the crystal lattice and is permanent in the sense that it is not in- 

 fluenced by the hydrogen ion concentration of the medium in which the 

 clay is suspended. There are also ' spots ' on the particles which are 

 charged or uncharged according to the reaction of the medium. They are 

 of two kinds : acidic spots where negative charges can develop through the 

 dissociation of hydrogen ions, and basic spots where positive charges can 

 develop through the combination of hydrogen ions. The process in the 

 case of the acidity spots is probably 



> Si - OH 5± > Si - O + H+ 



the silicon atoms being those situated at the edges of the silicon oxygen 

 sheets. The chemical nature of the basic spots is uncertain. They are 

 not found in the clay minerals so far identified but are frequent in the 

 common clays. The equilibrium is possibly 



- Al - OH ^ - Al = O + H+ 



and may be due to an over-crowding in the ' gibbsite ' layer. 



A study of the variation of the electric charge with pu. enables the 

 amounts of permanent charge and of the acidic and basic groups to be 

 determined. Approximate values have also been obtained for the dissocia- 

 tion constants of the groups concerned. In certain clays the number 

 of basic groups exceeds that of the permanent (negative) charges. These 

 exhibit well-defined isoelectric points. 



Prof. J. D. Bernal, F.R.S. — The hydroxyl bond in clay minerals 

 (11.30). 



The essential process that takes place in the formation of clays from 

 rock minerals such as felspar or mica is hydration. In the first stages, 

 however, water does not form part of the clay as such, but as hydroxyl 

 groups bound to magnesium, aluminium, or more rarely silicon ions. A 

 hydroxyl group bound to one of these ions is capable of attaching itself 

 to other hydroxyl or oxygen atoms in neighbouring layers owing to the 

 polarising power of the hydrogen it contains. The strength of the hydroxyl 

 bond thus formed depends on the charge of the ion to which the oxygen 

 atoms are attached. It is strongest for a silicon, weakest for a magnesium 

 ion, and this is also the order of the capacity to lose the hydrogen ion 

 altogether or the order of decreasing acidity of the clay particle. 



In greater degrees of hydration water molecules are bound to the hydroxyl 

 group, but in a way which resembles the structure of ice more than it does 

 that of free water, owing to the directing effect of the hydroxyls. 



General Discussion (11.50). 



