Subsolidus relations on the dolomite join, CaMg(CO3)2-CaFe(CO3)2-CaMn(CO3)2 and on bounding joins have been studied largely at 450°C and total pressures of 2-3 kbars using hydrothermal apparatus and cold-seal pressure vessels. Under these conditions, the dolomite join contains one single-phase, one three-phase and three two-phase areas.
Ternary relations exist only within a one-phase area which occupies most of the Mg-rich portion of the dolomite join; these solid solutions are believed to be cation-ordered. A two-phase area of disordered Mg-poor solid solutions extends almost the entire length of the join CaFe(CO3)2-CaMn(CO3)2; the hypothetical iron analog of dolomite, CaFe(CO3)2, which has neither been synthesized nor reported as a mineral, lies within this two-phase area. Two disordered solid solutions, one Ca-rich and the other Ca-poor coexist with dolomite solid solutions in a three-phase area located in the Mg-poor region and extending from the join CaMg(CO3)2-CaFe(CO3)2 to approximately 40 mole percent CaMn(CO3)2. Flanking the three-phase area and separating it from the one-phase area is a narrow two-phase area in which dolomite solid solutions coexist with disordered, Ca-poor solid solutions. The presence and extent of a third two-phase area has been inferred from subsolidus relations on the join CaMg(CO3)2-CaMn(CO3)2.
Application of phase relations on the dolomite join to natural carbonate assemblages is limited by the presence of excess CaCO3 in most natural dolomites.