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.

Aa 0 f B W -AH=O 0 A I a 0 .-E ,5Reaction between CaO powders and CO&) is often limited by slow diffusion of COz through the CaC03 product. Additions of Li2C03 are shown to increase the reaction rate. At temperatures around 940 K, near to the LiZCO3-CaCO3 eutectic temperature, 935 K, complete reaction of the CaO with COz can be achieved when the heating rate and Li2C03 content are optimized. At lower temperatures LizC03 causes lesser but measurable increases in reaction rates. SEM and surface area observations suggest different reaction paths when LizC03 is present, when LizC03 and eutectic are present, and when eutectic alone is present.

have been investigated by transmission electron microscopy.