Raman spectroscopy of the borosilicate mineral ferroaxinite

Abstract

Raman spectroscopy, complemented by infrared spectroscopy has been used to characterise the ferroaxinite minerals of theoretical formula Ca2Fe2+Al2BSi4O15(OH), a ferrous aluminium borosilicate. The Raman spectra are complex but are subdivided into sections based upon the vibrating units. The Raman spectra are interpreted in terms of the addition of borate and silicate spectra. Three characteristic bands of ferroaxinite are observed at 1082, 1056 and 1025 cm-1 and are attributed to BO4 stretching vibrations. Bands at 1003, 991, 980 and 963 cm-1 are assigned to SiO4 stretching vibrations. Bands are found in these positions for each of the ferroaxinites studied. No Raman bands were found above 1100 cm-1 showing that ferroaxinites contain only tetrahedral boron. The hydroxyl stretching region of ferroaxinites is characterised by a single Raman band between 3368 and 3376 cm-1, the position of which is sample dependent. Bands for ferroaxinite at 678, 643, 618, 609, 588, 572, 546 cm-1 may be attributed to the ν4 bending modes and the three bands at 484, 444 and 428 cm-1 may be attributed to the ν2 bending modes of the (SiO4)2-.