X-ray diffraction and spectroscopic study of SrxCa1−xCO3: Implications for equilibrium Sr2+ incorporation and carbon/oxygen isotope fractionation in aragonite

Abstract

Eight SrxCa1−xCO3 (0.2 < x < 0.9) solid solutions with orthorhombic aragonite structure were synthesized at P = 1.5 GPa and T = 1300 °C over 48 hours. Powder X-ray diffraction (XRD) analyses show that their unit-cell parameters and volumes increase systematically with increasing Sr2+ concentration, and the excess volume (ΔV) has been found to be insignificant at temperatures up to 700 K, or pressures up to 3 GPa. The Raman and Fourier-transform infrared (FTIR) spectra of these solid solutions show that all lattice and internal vibrational modes shift systematically to lower frequencies with increasing Sr2+ content, exhibiting a negative dependence of vibrational frequencies on the average mass of divalent cation for the aragonite-group carbonates in the order of aragonite > strontianite > witherite > cerussite. The vibrational contribution to the excess Gibbs free energy is constrained for the SrxCa1−xCO3 solid solution, whose miscibility gap shrinks at elevated temperatures and disappears above ~400 K. The calculated equilibrium Sr-Ca exchange coefficient (KDSr/Ca) between aragonite and seawater is systematically lower than the measured ones below ~80 °C, suggesting kinetic control on Sr partitioning at low temperatures. We also evaluate the metal cation effect on the carbon and oxygen isotope fractionations between aragonite-group carbonates and CO2 or H2O. Our results show that the 1000∙lnβ (where β is the reduced partition function ratio) for both carbon and oxygen isotope fractionations decrease linearly with increasing average atomic mass of the divalent cation in these aragonite-group carbonates, and the compositional effect on isotope fractionations can be treated as a linear function of Sr2+ concentration in the SrxCa1−xCO3 solid solution.