Synthetic magnesian calcites containing 4.94, 9.89, 14.83, and 19.78 mol per cent MgCO3 in solid solution were prepared by solid-state reaction of pelletized mixtures of CaCO3 and basic Mg carbonate at temperatures between 800° and 860° C. Spectrographic analyses of the Mg content of these solid solutions, using copper spark and d.c. are methods, gave results which were, with one exception, within ± 7 per cent of the true values.
Dolomite crystals from five localities were analyzed by wet chemical methods, and gave molar compositions ranging from Ca0.4940 (Mg, Fe, Mn)0.5060 to Ca0.5063 (Mg, Fe, Mn)0.4937.
Values of a0 and c0 for magnesian calcites, the dolomites, and a synthetic magnesite were obtained from x-ray diffractometer and film measurements. The spacings of d633(30.12), the third order of the strongest reflection, measured on the diffractometer agree with those computed from measured values of a0 and c0. The synthetic magnesite has a0 = 4.6330 Å, c0 = 15.016 Å, arh = 5.6752 Å, α = 48° 10.9′.
The average a0 for the dolomites studied is 0.092 per cent less than one-half the sum of a0 values for calcite and magnesite, and the average c0 value, 0.21 per cent less than the analogous hypothetical c0 value. If the magnesian calcite curves are extrapolated to 50 mol per cent MgCO3, the comparable numbers are 0.98 per cent for a0 and 0.44 per cent for c0. The implied smaller mole volume for the hypothetical disordered equivalent of dolomite must be viewed with skepticism, however, because of the length of the extrapolation and the fact that cell constants of the magnesian calcites were measured at room temperature, from 500° to 800° C. below the minimum temperature at which they are stable.
Compositions of single-phase materials between calcite and dolomite can be determined from c0 values to within about 2 mol per cent MgCO3 if mixed-layer effects do not occur.