Magnesium oxides are prepared by calcinating the fine plate-like basic magnesium carbonate and the coarse rod-like magnesium carbonate at 600°800°, 1000°,  1200°and 1400℃. The effects of the sizes and shapes of the carbonates and the effects of the calcination temperature on the properties of magnesias are studied by the observation with the electron microscope and by the measurements of iodine adsorptions, apparent densities and solubilities in the acetic acid.
The growth of the sizes of the primary particles and the decrease of the specific surfaces of the magnesias are occured with the raise of the calcination temperature. Above 1000℃., the characteristic cubic crystals of the magnesium oxide are observed. The sintering of the magnesia particles takes place above 1200℃.
The primary particles of the magnesia form a kind of skeletons, i. e. the secondary particles, and they retain the sizes and shapes of the original carbonates, therfore the apparent densities of the magnesias are close to that of the carbonates from which they were prepared.