Crystal structure of calcite-type Ca1–xMnxCO3 solid solution by X-ray diffraction and Raman spectroscopy

To investigate the quantitative relationship between the crystal structure and composition of Mn-bearing calcite, the solid solutions of Ca_1–xMn_xCO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) with continuous MnCO₃ mol% content were synthesized at 1 GPa and 700 °C using high-purity CaCO₃ and MnCO₃ powders as starting materials. The run products were analysized by electron probe, powder X-ray diffraction and Raman spectroscopy. The CaO wt% and MnO wt% of the resulting products are consistent with the expected compositions. The powder X-ray diffraction results show that the products are single phase without any impurities. All diffraction peaks of samples with varying MnCO₃ mol% contents can be indexed by the calcite-type structure carbonates ACO₃ (R-3c space group; A is a divalent cation), confirming the previous results that there is the completely continuous solid solution between CaCO₃ and MnCO₃ end members. The unit-cell parameters and volumes of the solid solutions decrease as the MnCO₃ mol% content increases, presenting a linear relationship of Ca–Mn ideal miscibility, which is perfectly consistent with the rigid body model of A-site substitution in ACO₃. Besides, as MnCO₃ mol% content increases, the bond distance of A–O decreases linearly, while the bond distance of C–O changes like a parabola. Therefore, the addition of Mn makes the bond distance of A–O shorten, resulting in the decrease of unit-cell parameters and volumes for Ca_1–xMn_xCO₃. Furthermore, two exterior vibrations (T and L) of the crystal lattice and two internal vibrations (ν₄ and ν₁) within the CO₃²⁻ unit are assigned in the Raman spectra of these solid solutions. The characteristic vibration modes T, L, and ν₄ as a whole increase with the increasing of MnCO₃ mol% content, whereas the characteristic vibration mode ν₁ as a whole decreases with the increase of MnCO₃ mol% content. These variations in Raman vibration modes are related to the radius of substituted ions.