The join CaCO₃-MgCO ₃ is one of the most important rock-forming binary carbonate systems. Knowledge of the Ca-Mg carbonate relation is fundamental to the understanding of sedimentary, marine biological and geochemical, metamorphic and mantle processes. The configuration of the CaCO₃-MgCO₃ phase diagram is dependent on the nature of the order – disorder phase transition in samples at intermediate compositions (dolomites [CaMg(CO₃)₂]). Samples close to the calcite and magnesite end-members have  symmetry, but the ordered phase at intermediate compositions displays  symmetry. Disordered dolomites can be found in nature where they form metastably at low temperature, or can be obtained by annealing ordered material at high temperatures. Heat treatment above ~1200 °C causes cation disordering and ordered dolomites convert to the  structure. However, the high temperatures involved and possible problems in quenching the disordered phase make detailed study of the order – disorder transition in dolomites difficult. The CdCO₃-MgCO ₃ system was quantitatively investigated as an aid to understanding the petrological importance of calcite – dolomite – magnesite relations. The phase diagram of the Cd-Mg join, in fact, appears to model that of the Ca-Mg system, but at significantly lower temperatures. Moreover, the ordered phase at intermediate compositions presents a larger stability field.

In order to study the macroscopic (by means of X-ray diffraction) and microscopic (by means of IR spectroscopy) behaviour of this solid solution, samples are being synthesised in rapid quench hydrothermal pressure vessels, using silver oxalate as a CO₂ source to prevent dissociation of carbonates. Experiments are being carried out at 600 °C and 850 °C and 100 MPa using appropriate mixtures of MgCO₃ and CdCO₃ as starting materials in order to obtain two sets of samples with compositions between the two end-members and different degrees of order. In fact, whereas at 600 °C the carbonates at intermediate compositions have a lower symmetry with respect to the end-members due to the ordering of Mg and Cd, at 850 ° C the whole solid solution has  symmetry. Comparison between the two sets of samples will thus give insight into the cation ordering process in these carbonates.