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

•The vibrational spectroscopy of many borate minerals is ill determined.•Included is hydroboracite CaMg[B3O4(OH)3]2·3H2O.•The basic repeat unit is the [B3O4(OH)3]2− group consisting of 2 tetrahedra and 1 triangle.•The mineral is characterized by a Raman band at 1039 cm−1.

We have studied the mineral hydroboracite CaMg[B3O4(OH)3]2·3H2O 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) × (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−1are 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.