Dolomite (MgCO3CaCO3) is identified as the crystalline phase in the low temperature range, i.e. at temperatures less than 816 K. At 816 K, the high temperature compound periclase (MgO) is produced, as a consequence of, phase transition. This is confirmed from iso-intensity plot of DTXRD. For MgO at 1273 K, the cubic unit cell parameter is estimated to be about 4.26 A°. We observe certain microscopic changes on dolomite at temperatures less than 816 K as is evidenced from diffraction patterns.