Dedicated to Professor Karl Jasmund at the Occasion of His Retirement

While much attention has been paid to the structural changes which accompany the dehydration of kaolinite (Brindley & Nakahira, 1959; Freund, 1967, 1972; Chakraborty, 1979), the early stages of dehydration have not been studied so extensively (Fripiat & Toussaint, 1963). One question of quite broad interest is that of proton mobility. The question arises from the seemingly paradoxial observation that protons are quite tightly bound in the O H ions but H20 molecules are easily formed during dehydration. As thermodynamic values (Vedeneyev et al., 1966) tell us, the O -H bond energy amounts to about 110 kcal mole -1 (4.1 eV). From spectroscopic studies (Martens & Freund, 1976) it is known that in Mg(OH)2, which can be considered as a model ionic hydroxide, the potential energy curve separating adjacent proton sites rises to about 50 kcal mole-1 (2 eV). As judged by its structure and by its IR spectrum in the O -H stretching region, kaolinite can also be classified as an ionic hydroxide with energetically well-separated O H ions. If dehydration of kaolinite occurs above 450~ half of the H + must leave their parent hydroxyl ions to form O 2and recombine with the other half of the O H to form H20 molecules. In view of the tight O-H bond-about 110 kcal mole1 for the free O H and as high as 50 kcal mole I for the bond state in hydroxides--this proton delocalization is by no means a trivial process. The question arises--what pathways are available for the protons to become mobile? Protons, regarded as a chemical entity, are carriers of a positive charge. Their delocalization process may be studied by electrical conductivity measurements. Fripiat & * On leave from: Fertilizer (Planning and Development Division) India Ltd., Sindri, India. 0009-8558/81/1200-0395502.00 9 1981 The Mineralogical Society 396 G. C. Mai t i and F. Freund Toussaint (1963) used electrical conductivity methods in vacuum and observed current effects which were explained by proton delocalization; the possible conductivity mechanisms were not clarified, however. In the present paper we report specific proton conductivity measurements in conjunction with IR measurements. H O W TO M E A S U R E P R O T O N C O N D U C T I V I T Y Alternating current conductivity Any OH -containing compound is potentially a proton conductor. Its conductivity can be measured by either alternating current (a.c.) or direct current (d.c.) methods. A.c. measurements place no special conditions upon the electrodes to be used: all mobile charge carriers in a given sample will move to and fro in a given alternating electric field without necessarily touching the electrodes. Oscillating charge carriers extract energy from the a.c. field and thus cause dielectric losses. These can be expressed as a phase shift between the incoming and the outgoing a.c. field, i.e. tan 6 (Daniel, 1967; Gieseke et al., 1970). From the frequency dependence of the tan 5 maxima, valuable information can be drawn about the charge carriers, in particular