Raman spectroscopy of newberyite Mg(PO 3OH)·3H 2O: A cave mineral

Abstract

Newberyite Mg(PO 3OH)·3H 2O is a mineral found in caves such as from Moorba Cave, Jurien Bay, Western Australia, the Skipton Lava Tubes (SW of Ballarat, Victoria, Australia) and in the Petrogale Cave (Madura, Eucla, Western Australia). Because these minerals contain oxyanions, hydroxyl units and water, the minerals lend themselves to spectroscopic analysis. Raman spectroscopy can investigate the complex paragenetic relationships existing between a number of ‘cave’ minerals. The intense sharp band at 982 cm −1 is assigned to the PO 4 3− ν 1 symmetric stretching mode. Low intensity Raman bands at 1152, 1263 and 1277 cm −1 are assigned to the PO 4 3− ν 3 antisymmetric stretching vibrations. Raman bands at 497 and 552 cm −1 are attributed to the PO 4 3− ν 4 bending modes. An intense Raman band for newberyite at 398 cm −1 with a shoulder band at 413 cm −1 is assigned to the PO 4 3− ν 2 bending modes. The values for the OH stretching vibrations provide hydrogen bond distances of 2.728 Å (3267 cm −1), 2.781 Å (3374 cm −1), 2.868 Å (3479 cm −1), and 2.918 Å (3515 cm −1). Such hydrogen bond distances are typical of secondary minerals. Estimates of the hydrogen-bond distances have been made from the position of the OH stretching vibrations and show a wide range in both strong and weak bonds.