Disorder and Mg concentration effects on equilibrium C-O isotope fractionations in MgxCa1-xCO3 solid solutions (0 ≤ x ≤ 0.5): A crystallographic and spectroscopic study of magnesian calcite and dolomite

Abstract

Magnesian calcites and dolomite are critical carbonate minerals that archive valuable environmental and geochemical records. In this study, MgxCa1-xCO3 solid solutions (0 ≤ x ≤ 0.5) were synthesized at 1100 °C and 1.5 GPa using mixtures of natural calcite and dolomite as starting materials. Their structural and vibrational properties were analyzed using single-crystal X-ray diffraction (XRD) technique, Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The homogeneity of the products was checked by electron microprobe analysis (EMPA) and XRD on multiple crystals, and the minor compositional variations detected on sample chips do not measurably affect lattice parameters or vibrational modes. The synthesized magnesian calcites adopt the calcite-type structure (R3¯c). Dolomite, initially refined in R3¯ symmetry, undergoes an order–disorder transition upon heating, with the order parameter (s) decreasing from 0.79 to 0.02. Increasing Mg2+ content linearly reduces the unit-cell volume and average Ca(Mg)-O bond lengths, while all major vibration modes shift to higher frequencies. More interestingly, we show that vibration frequencies of MgxCa1-xCO3 (0 ≤ x ≤ 0.5) solid solutions are volume-controlled, with lattice compression governing the shifts of the vibration frequencies rather than C-O bond shortening. Equilibrium 103·lnβ values for carbon and oxygen isotope fractionations, calculated from the observed vibration frequencies, rise linearly with Mg2+ substitution at a given temperature. Using these β factors, equilibrium carbon and oxygen isotope fractionation factors are further calculated for calcite/dolomite − CO2/H2O systems and compared with previous experimental data. Our results show that thermally-induced structural disorder expands the unit-cell volume of dolomite by ∼0.4 % at ambient conditions, reducing 103·lnβ values. The ab initio calculations confirm these reductions due to the disorder effect, with 103·lnβ values decreasing by ∼0.8 ‰ at T = 300 K for both C and O isotope fractionations. Our results quantify the dual controls of Mg2+ substitution and structural disorder on equilibrium oxygen and carbon isotope fractionation in magnesian calcite-group minerals, resolving inconsistencies between earlier experimental results and theoretical calculations. The insights gained from our findings broaden the potential applications of C-O isotope fractionation in both laboratory and natural settings.