Kutnohorite CaMn(CO3)2 crystal growth at high pressure-temperature

High quality kutnohorite CaMn(CO3)2 single crystals up to 100 μm in size were successfully achieved under high pressure-temperature (P-T) conditions. Electron microprobe analyses revealed the average wt% of CaO was 25.98% and that of MnO was 32.88%, correspondingly well to the ideal formula of Ca1.0Mn1.0(CO3)2. Accurate crystalline structural data were determined from single-crystal X-ray diffraction (XRD), with the R3 space group and R3c space group used to refine the crystal structure of CaMn(CO3)2. The Ca-O and Mn-O bond lengths were slightly different when using the R3 space group, which were clearly distinguished from those in the dolomite structure. Therefore, R3c is the most probable space group for the CaMn(CO3)2 crystal structure. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) showed that CaMn(CO3)2 decomposed from 620–750 °C, but only one endothermic peak was observed during the decomposition process. It indicated that the octahedral units in CaMn(CO3)2 have the same thermal stability due to the complete miscibility of Ca and Mn. The results of single crystal XRD and thermal analysis provided direct evidence that CaMn(CO3)2 has a calcitetype structure, not dolomite-type layered structure, which was in good agreement with the rigid model of rhombohedral carbonates.