Crystal structure and vibration spectra of a norsethite-type BaMg(CO₃)₂–BaMn(CO₃)₂ solid solution

Abstract Members of norsethite-type carbonate solid solutions with the compositions Ba(Mg 1– x Mn x )(CO 3 ) 2 , ( x = 0, 0.25, 0.50 and 0.75) have been synthesised under high-pressure and -temperature conditions (3GPa, 800°C) for the first time. The synthetic transparent crystals gradually changed their appearance from colourless to blue lustre with the increasing Mn 2+ content (X Mn ). The results of the crystal structure analyses reveal that the lattice parameters ( a, c , unit-cell volume, Mg/Mn–O bond lengths and Ba–O bond lengths) complied with a linear increase with X Mn . In contrast, the C–O bond lengths and O–C–O bond angles decreased, because the CO 3 2– group was squeezed by the expansion of the (Mg/Mn)O 6 octahedra. Moreover, the Raman and infrared vibrations, except for the lattice mode T , shift to low frequency with the increasing X Mn , and the slight corresponding variations of the atomic positions were also determined. These new results demonstrate the impact of Mg 2+ –Mn 2+ substitution on the crystal chemistry of norsethite-type solid solutions, with further implications for the natural occurrence and environmental of norsethite-type and dolomite/ankerite-type carbonates.