Mechanism of decomposition of dolomite, Ca_0.5Mg_0.5CO₃, in the electron microscope

Decomposition of dolomite, Ca_0.5Mg_0.5CO₃, induced by the electron beam in the transmission electron microscope, was studied by means of electron diffraction patterns and high-resolution images. Radiolysis of the carbonate ion gives a metastable, fcc solid solution, Ca_0.5Mg_0.5O, which is formed by a topotactic process so that a body diagonal of the fcc unit cell is coincident with the original dolomite c-axis. The fcc lattice constant of 0.461 ± 0.006 nm is 2% larger than the mean value for CaO and MgO. Subsequently, the fcc domains lose orientation, the specimen passes through an amorphous state, and randomly oriented crystallites of CaO and MgO are formed. The diameters of the crystallites of both the solid solution and the final CaO and MgO are in the range 1 to 10 nm. Thermal decomposition in vacuum differs from electron-beam decomposition in that the range of solid-solution compositions is much more limited in the former case. Electron-beam decomposition of minerals may be a useful method of preparation of highly reactive solid solutions that are not otherwise obtainable.