The crystal structure of synthetic BaMg(CO3)2 whose mineral name is norsethite was re-investigated by single-crystal X-ray diffraction. Complementary in situ high- and low-temperature studies by means of vibrational spectroscopy (Raman, IR), powder X-ray diffraction techniques and thermal analyses were performed. Diffraction images (298 K) revealed weak superstructure reflections caused by the displacement of the O atoms in the earlier considered Rm structure model (a = 5.0212(9), cnew = 2 cold = 33.581(6) Å, Rc, Z = 6, R1 = 0.011, sinθ/λ < 0.99 Å−1). Thermal analyses reveal decarbonatization in two decomposition steps above 750 K, and the heat-flow curves (difference scanning calorimetry) give clear evidence of a weak and reversible endothermal change at 343±1 K. This agrees with a discontinuity in the IR and single-crystal Raman spectra. The changing trend of the c/a ratio supports this discontinuity indicating a temperature-induced structural transition in the range between 343 and 373 K. As the change of the unit-cell volume is almost linear, the character of the transition is apparently second order and matches the mechanism of a subtle displacement of the oxygen atom position. The apparent instability of the Rc structure is also evidenced by the remarkably larger anisotropic displacement of the oxygen atom.

You do not currently have access to this article.