The surface hydration characteristics of high-surface-area magnesium oxide prepared by decomposition of the hydroxide have been studied by sorption and infra-red techniques. The results are consistent with a simple model for HzO chemisorption on the MgO (100) face. Dehydration at -1OOO"C modifies the nature of the surface in a manner which is only reversed under drastic rehydration conditions corresponding to the adsorption of multilayers of water, but the dehydrated magnesia surface does not exhibit the resistance to rehydration characteristic of many other oxides. On first formation of MgO from the hydroxide, the normal oxide surface structure is formed only very slowly and a metastable surface is readily distinguished in the infra-red spectra; the reformation of this intermediate surface structure seems to be a necessary stage in the bulk rehydration of oxide to the hydroxide.

The decomposition of a synthetic Mg(OH)2 powder and two natural brucite minerals has been investigated. Reaction 's found to proceed by the advance of a two-dimensional reaction interface, at a linear rate, from tie perimeter to the centre of the basal plane of the hydroxide crystal. A kinetic model based on this result accounts for the effect of crystal size and shape upon the decomposition kinetics and leads to pre-exponential factors all within 4 . 5 to 2 5 x 1010 sec-1 for the three reactants which differ in surface area by a factor -105.The whole reaction is envisaged as occurring in three stages, not all of which may be distinaished experimentally for a given reactant.

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Isosteric heats of adsorption of argon on MgO smoke and on five samples produced from Mg(OH), have been determined from adsorption isotherms. H e a t s of adsorption of the isolated atom at various positions on different surface planes, in very small intercrystallite pores and at position of '' contact " of adjacent crystals, have been calculated theoretically. The theoretical curve of isosteric heat as a function of adsorbate concentration on a plane (100) surface is close to the experimental curve for the MgO smoke above -20 % coverage. The microcrystalline oxides prepared from Mg(OH)2 yield higher heats of adsorption. From the theoretical calculations this is not explicable as adsorption on surfaces other than (100) nor in terms of " surface heterogeneities ", but is consistent with adsorption in very small pores or at points of crystal-crystal contact, where the adsorbate atom is close to more lattice ion neighbours than on the plane surface. The variations in heats of adsorption between the different samples are consistent with the changes in microstructure brought about by different calcination treatments.

Two microporous Be0 powders which differed only in that in one the mean pore width was < 6 8, and in the other was -6-20 8,, were used to study the effect of molecular size pores upon the micro-structural changes during calcination. As with macroporous oxides, the mean pore width increased with increasing calcination temperature and pores < 6 A width were eliminated by -500°C.Since significant crystal growth in V ~C U O is not observed at such low temperatures, the initial presence of these very small pores has no significant effect upon crystal growth during calcination at high temperatures. The elimination of pores of width < 6 A is accompanied by a decrease in intraaggregate pore volume (which is not observed on elimination of larger micropores) and this leads to a lower temperature of aggregate densification for the oxide containing the smallest pores. In the presence of H 2 0 vapour the processes of pore and crystal growth occur at significantly lower temperatures than in uacuo but aggregate densification is not enhanced.