Vibrational spectroscopic characterization of the phosphate mineral althausite Mg2(PO4)(OH,F,O) – Implications for the molecular structure

• We have studied the mineral althausite Mg2(PO4)(OH,F,O).
• It occurs as a secondary product in granitic pegmatites.
• Raman and infrared bands are assigned to HOPO33- and PO43- units.
• No bands which are attributable to water were found.
• Vibrational spectroscopy enables aspects of the molecular structure of althausite.

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 ν1 symmetric stretching modes of the HOPO33- and PO43- 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 ν4 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 ν2 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.

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