Raman and mid-IR spectroscopic study of the magnesium carbonate minerals - brugnatellite and coalingite

Two hydrated hydroxy magnesium carbonate minerals brugnatellite and coalingite with a hydrotalcite-like structure were studied by Raman spectroscopy. Intense bands are observed at 1094 cm(-1) for brugnatellite and at 1093 cm(-1) for coalingite attributed to the CO32- v(1) symmetric stretching mode. Additional low intensity bands are observed at 1064 cm(-1). The existence of two symmetric stretching modes is accounted for in terms of different anion structural arrangements. Very low intensity bands at 1377 and 1451 cm(-1) are observed for brugnatellite, and the Raman spectrum of coalingite displays two bands at 1420 and 1465 cm(-1) attributed to the (CO3)(2-) v(3) antisymmetric stretching modes. Very low intensity bands at 792 cm(-1) for brugnatellite and 797 cm(-1) for coalingite are assigned to the CO32- out-of-plane bend (v(2)). X-ray diffraction studies by other researchers have shown that these minerals are disordered. This is reflected in the difficulty of obtaining Raman spectra of reasonable quality and explains why the Raman spectra of these minerals have not been previously or sufficiently described. A comparison is made with the Raman spectra of other hydrated magnesium carbonate minerals. Copyright (C) 2008 John Wiley & Sons, Ltd.