The progressive crystallization and ordering of low-temperature dolomites

The progressive crystallization of dolomite has been investigated in the temperature range of 90 to 41o ~ by means of X-ray powder diffraction, electron microscopy, and infra-red spectroscopy. Short-time, low-temperature experiments ( ~ 145 ~ yielded dolomites with a high defect density. In detail, lattice faults can be described as: Random succession of more or less ordered cation domains, producing a long-range mosaic-type disorder; cation ordering may take place along and < ~ I~o); tilting and dislocation of individual COa groups; and irregular interstratification of dolomite layers of different chemical compositions in the crystallographic c-directions. Dolomites produced in experiments of longer duration are composed of an interstratification of essentially two chemically different dolomite layers, of which the stacking sequence is more perfect than it is at lower crystallization degrees. Both components grow rapidly at the expense of dolomites of intermediate composition. Finally, long-term low-temperature (145 ~ experiments produced two independent, coexistent dolomite phases. Within single dolomite layers cations now lie very close to their theoretical positions. COn-tilting and dislocation decreases markedly. Hydrothermal runs at temperatures > c. 145 ~ yielded one single, nearly stoichiometrie, highly ordered dolomite phase. Finally, dolomites synthesized at temperatures > c. 200 ~ are of an ideal chemical composition and have perfect lattice ordering.