Key points are not available for this paper at this time.
Isothermal sections at 600°, 650°, 700°, 750°, and 800° C, run for the most part at 15 kilobars total pressure, were obtained using a squeezer-type apparatus. Above approximately 675° C. the system contains two two-phase and two single-phase areas. One single phase region extends a short distance into the ternary system along the full length of the MgCO3-FeCO3 binary join. The other extends as a thin sliver from dolomite toward the composition CaFe(CO3)2, curving toward and enlarging into the CaCO3 apex after traversing approximately two-thirds of the distance. A three-phase area in the Mg-poor portion of the system is first detected at 675° C. and becomes progressively larger at 650° and 600° C. Ferroan dolomite, ferroan-magnesian calcite, and calcian-magnesian siderite coexist in this three-phase triangle. The hypothetical compound CaFe(CO3)2, not known in nature, lies in a two-phase field even at 800° C. The maximum FeCO3 substitution observed in twenty naturally occurring ferroan dolomites examined is 31 mol %, essentially the same as the maximum equilibrium substitution achieved experimentally. Both the natural and synthetic single-phase ferroan dolomites contain up to 6 mol % excess O3. The FeCO3 content of natural ferroan dolomites cannot at this time be used for geological thermometry because such dolomites may be compositionally metastable. A spectrochemical method is described which, for a typical ferroan dolomite containing 53 mol % O3, 31 mol % MgCO3, 15 mol % FeCO3, and 1 mol % MnO3, determines these quantities with estimated root mean square relative errors corresponding to ±1.6 mol % CaCO3, ±1.2 mol % MgCO3, ±1.0 mol % FeCO3, and ±0.06 mol % MnCO3. The rate of change of a0 with composition among the ferroan dolomites is only about two-thirds that predicted by interpolation among the a0 values for MgCO3, and FeCO3; that of c0 is about half.
Goldsmith et al. (Thu,) studied this question.