Vibrational spectroscopic characterization of the phosphate mineral barbosalite [formula omitted] – Implications for the molecular structure

Abstract

Natural single-crystal specimens of barbosalite from Brazil, with general formula Fe2+Fe23+(PO4)2(OH)2 were investigated by Raman and infrared spectroscopy. The mineral occurs as secondary products in granitic pegmatites. The Raman spectrum of barbosalite is characterized by bands at 1020, 1033 and 1044cm−1cm−1, assigned to ν1 symmetric stretching mode of the HOPO33- and PO43- units. Raman bands at around 1067, 1083 and 1138cm−1 are attributed to both the HOP and PO antisymmetric stretching vibrations. The set of Raman bands observed at 575, 589 and 606cm−1 are assigned to the ν4 out of plane bending modes of the PO4 and H2PO4 units. Raman bands at 439, 461, 475 and 503cm−1 are attributed to the ν2 PO4 and H2PO4 bending modes. Strong Raman bands observed at 312, 346cm−1 with shoulder bands at 361, 381 and 398cm−1 are assigned to FeO stretching vibrations. No bands which are attributable to water vibrations were found. Vibrational spectroscopy enables aspects of the molecular structure of barbosalite to be assessed.