The Langmuir method for measurement of vapour pressures has been tested for use in studies of decomposition reactions. The isothermal weight loss in vacuum from cleavage (1011) planes of calcite (CaC03) single crystals was measured continuously at temperatures from 934 to 1013 K. The rate was constant until approximately 80 % of a 1 mm slice had decomposed. The apparent activation enthalpy was 205 kJ (49 kcal) mol-'. Micrographic examination showed an approximately 30 pm thick layer, probably a metastable form of calcium oxide, separating the calcite from the growing layer of oriented stable calcium oxide. The 30 pm layer yielded a single X-ray diffraction peak which was displaced slightly from the strongest (220) peak of the oriented normal calcium oxide. Lower apparent activation enthalpies measured in previous studies of calcite decomposition in inert atmospheres are suggested to result either from partial diffusion control of the process or from catalysis of the direct formation of normal calcium oxide by carbon dioxide or a component of the system atmosphere. The ratio of the measured decomposition rate in vacuum to the maximum rate, which can be calculated from the Hertz-Knudsen-Langmuir equation, is suggested to be a useful parameter in correlating and predicting decomposition reaction rates.

A transmission electron microscopy study has been carried out to understand the structural transformation mechanisms of decomposition of Mg(OH)2 and MgC03 under vacuum. Both Mg(OH)2 and MgC03 decompose topotactically to yield porous pseudomorphic MgO of normal structure with definite orientation relationships. The decomposition of Mg(OH)2 yields MgO with a single orientation relationship, but that of MgC03 leads to one major and two minor orientation relationships with two, six, and three variants, respectively. The major orientation relationships in the formation of 2-to 3-nm cubic MgO particles are discussed in terms of a correlation between oxygen octahedra in the reactant solids. The small cubic MgO particles may aggregate spontaneously to reduce the excess surface energy and result in minute cracks.

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