Vibrational spectroscopic characterization of the phosphate mineral althausite Mg2(PO4)(OH,F,O) - implications for the molecular structure.

Abstract

Natural single-crystal specimens of althausite from Brazil, with general formula Mg2(PO4)(OH,F,O) were investigated by Raman and infrared spectroscopy. The mineral occurs as a secondary product in granitic pegmatites. The Raman spectrum of althausite is characterized by bands at 1020, 1033 and 1044 cm_1, assigned to m1 symmetric stretching modes of the HOPO3_ 3 and PO3_ 4 units. Raman bands at around 1067, 1083 and 1138 cm_1 are attributed to both the HOP and PO antisymmetric stretching vibrations. The set of Raman bands observed at 575, 589 and 606 cm_1 are assigned to the m4 out of plane bending modes of the PO4 and H2PO4 units. Raman bands at 439, 461, 475 and 503 cm_1 are attributed to the m2 PO4 and H2PO4 bending modes. Strong Raman bands observed at 312, 346 cm_1 with shoulder bands at 361, 381 and 398 cm_1 are assigned to MgO stretching vibrations. No bands which are attributable to water were found. Vibrational spectroscopy enables aspects of the molecular structure of althausite to be assessed.