Relation between lattice constants and composition of the Ca-Mg carbonates

Abstract

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 cacoa and basic Mg carbonate at temperatures between 800o and 860o c. Spectrographic analyses of the Mg content of these solid solutions, using copper spark and tJ.c. arc 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 cao.rso (Mg, Fe, Mn)o.sootoo cao roor( Mg, Fe, Mn)o reaz. Values of ao and, co for magnesian calcites, the dolomites, and a synthetic magnesite were obtained from r-ray diffractometer and film measurements. The spacings oI d.636np ), the third order of the strongest reflection, measured on the diffractometer agree with those computed from measured v-alues of ao and co. The synthetic magnesite has oo:4.6330 A, ro:15.016 A, drr,:5.6752 A, a:48" 1 0 . 9 ' . The average os for the dolomites studied is 0.092 per cent less than one-half tne sum of as values for calcite and magnesite, and the average d0 value, 0.21 per cent less than the analogous hypothetical ,0 value. rf the magnesian calcite curves are extrapolated to 50 mol per cent MgCO3, the comparable numbers are 0.98 per cent for oo and 0.44 per cent for cs. 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 c6 values to within about 2 mol per cent Mgco3 if mixedJayer efiects do not occur.