Raman spectroscopy of the borate mineral ameghinite NaB3O3(OH)4

The molecular structure of the sodium borate mineral ameghinite NaB3O3(OH)4 has been determined by the use of vibrational spectroscopy. The crystal structure consists of isolated [B3O3(OH)4]− units formed by one tetrahedron and two triangles. H bonds and Na atoms link these polyanions to form a three-dimensional framework. The Raman spectrum is dominated by an intense band at 1027 cm−1, attributed to BO stretching vibrations of both the trigonal and tetrahedral boron. A series of Raman bands at 1213, 1245 and 1281 cm−1 are ascribed to BOH in-plane bending modes. The infrared spectra are characterized by strong overlap of broad multiple bands. An intense Raman band found at 620 cm−1 is attributed to the bending modes of trigonal and tetrahedral boron. Multiple Raman bands in the OH stretching region are observed at 3206, 3249 and 3385 cm−1. Raman spectroscopy coupled with infrared spectroscopy has enabled aspects about the molecular structure of the borate mineral ameghinite to be assessed.

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► Ameghinite NaB3O3(OH)4 is a mineral found in borax deposits.
► We have used the Raman and infrared spectrum to determine the molecular structure of ameghinite.
► Multiple OH stretching Raman bands are observed.