Vibrational spectra of BaMn(CO3)2 and a re-analysis of the Raman spectrum of BaMg(CO3)2

Raman and mid-infrared (MIR) spectra have been measured on BaMn(CO3)(2), an ordered double carbonate, synthesised at low and at high pressures and temperatures (1 atm, 21 degrees C and 15 kbar, 510 +/- 10 degrees C, respectively). The results show no significant influence of synthesis conditions on the vibrational spectra of the double carbonate within analytical uncertainty. This indicates no enhancement in positional disorder of the carbonate group in the low-temperature phase compared to the high-temperature phase. In addition, a re-analysis of the Raman spectrum of synthetic BaMg(CO3)(2) (norsethite) was carried out. Both double carbonates are structurally very similar and show close relationships in their spectra. One prominent difference is the occurrence of a sharp single v2 Raman mode in the spectrum of BaMg(CO3)(2), indicating some structural differences. A comparison of results obtained for the synthetic carbonates with literature data for solid-solutions between the two double carbonates (so-called 'Mn-bearing norsethites') suggests that the absorption band maxima of the mid-IR v1 and v3 vibrations follow linear trends as a function of the Mn index (X-Mn = Mn2+/(Mn2++Mg2+)) between the two end-member double carbonates, which may be explained by the different radii and masses of divalent magnesium and manganese.