A vibrational spectroscopic study of the borate mineral ezcurrite Na4B10O17·7H2O – Implications for the molecular structure

Abstract

We have studied the boron containing mineral ezcurrite Na4B10O17·7H2O 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 2cm−1 and as such is sufficient enough to identify separate bands for the stretching bands of the two boron isotopes. The Raman band at 1037cm−1 is assigned to BO stretching vibration. Raman bands at 1129, 1163, 1193cm−1 are attributed to BO stretching vibration of the tetrahedral units. The Raman band at 947cm−1 is attributed to the antisymmetric stretching modes of tetrahedral boron. The sharp Raman peak at 1037cm−1 is from the 11-B component such a mode, then it should have a smaller 10-B satellite near (1.03)×(1037)=1048cm−1, and indeed a small peak at 1048 is observed. The broad Raman bands at 3186, 3329, 3431, 3509, 3547 and 3576cm−1 are assigned to water stretching vibrations. Broad infrared bands at 3170, 3322, 3419, 3450, 3493, 3542, 3577 and 3597cm−1 are also assigned to water stretching vibrations. Infrared bands at 1330, 1352, 1389, 1407, 1421 and 1457cm−1 are assigned to the antisymmetric stretching vibrations of trigonal boron. The observation of so many bands suggests that there is considerable variation in the structure of ezcurrite. Infrared bands at 1634, 1646 and 1681cm−1 are assigned to water bending modes. The number of water bending modes is in harmony with the number of water stretching vibrations.