Infrared and Raman spectroscopic characterization of the borate mineral hydroboracite CaMg[B3O4(OH)3]2•3H2O - Implications for the molecular structure

We have studied the mineral hydroboracite CaMg[B3O4(OH)3](2)center dot 3H(2)O using electron microscopy and vibrational spectroscopy. Both tetrahedral and trigonal boron units are observed. The nominal resolution of the Raman spectrometer is of the order of 2 cm(-1) and as such is sufficient enough to identify separate bands for the stretching bands of the two boron isotopes. The Raman band at 1039 cm(-1) is assigned to BO stretching vibration. Raman bands at 1144, 1157, 1229, 1318 cm(-1) are attributed to the BOH in-plane bending modes. Raman bands at 825 and 925 cm(-1) are attributed to the antisymmetric stretching modes of tetrahedral boron. The sharp Raman peak at 925 cm(-1) is from the 11-B component such a mode, then it should have a smaller 10-B satellite near (1.03) x (925) = 952 cm(-1), and indeed a small peak at 955 is observed. Four sharp Raman bands observed at 3371, 3507, 3563 and 3632 cm(-1) are attributed to the stretching vibrations of hydroxyl units. The broad Raman bands at 3076, 3138, 3255, 3384 and 3551 cm(-1) are assigned to water stretching vibrations. Infrared bands at 3367, 3505, 3559 and 3631 cm(-1) are assigned to the stretching vibration of the hydroxyl units. Broad infrared bands at 3072 and 3254 cm(-1) are assigned to water stretching vibrations. Infrared bands at 1318, 1349, 1371, 1383 cm(-1) are assigned to the antisymmetric stretching vibrations of trigonal boron. (C) 2013 Elsevier B.V. All rights reserved.