Raman, infrared and near-infrared spectroscopic characterization of the herderite–hydroxylherderite mineral series

Abstract

Natural single-crystal specimens of the herderite–hydroxylherderite series from Brazil, with general formula CaBePO4(F,OH), were investigated by electron microprobe, Raman, infrared and near-infrared spectroscopies. The minerals occur as secondary products in granitic pegmatites. Herderite and hydroxylherderite minerals show extensive solid solution formation. The Raman spectra of hydroxylherderite are characterized by bands at around 985 and 998cm−1, assigned to ν1 symmetric stretching mode of the HOPO33- and PO43- units. Raman bands at around 1085, 1128 and 1138cm−1 are attributed to both the HOP and PO antisymmetric stretching vibrations. The set of Raman bands observed at 563, 568, 577, 598, 616 and 633cm−1 are assigned to the ν4 out of plane bending modes of the PO4 and H2PO4 units. The OH Raman stretching vibrations of hydroxylherderite were observed ranging from 3626cm−1 to 3609cm−1. The infrared stretching vibrations of hydroxylherderites were observed between 3606cm−1 and 3599cm−1. By using a Libowitzky type function, hydrogen bond distances based upon the OH stretching bands were calculated. Characteristic NIR bands at around 6961 and 7054cm−1 were assigned to the first overtone of the fundamental, whilst NIR bands at 10,194 and 10,329cm−1 are assigned to the second overtone of the fundamental OH stretching vibration. Insight into the structure of the herderite–hydroxylherderite series is assessed by vibrational spectroscopy.