IngentaConnect Raman and infrared spectroscopic study of the vivianite-group pho...

Authors: R.L. Frost; W. Martens; P.A. Williams; J.T. Kloprogge

Source: Mineralogical Magazine, Volume 66, Number 6, 1 December 2002 , pp. 1063-1073(11)

Abstract:

The molecular structure of the three vivianite-structure, compositionally related phosphate minerals vivianite, baricite and bobierrite of formula M²₃⁺(PO₄)₂.8H2O where M is Fe or Mg, has been assessed using a combination of Raman and infrared (IR) spectroscopy. The Raman spectra of the hydroxylstretching region are complex with overlapping broad bands. Hydroxyl stretching vibrations are identified at 3460, 3281, 3104 and 3012 cm¹ for vivianite. The high wavenumber band is attributed to the presence of FeOH groups. This complexity is reflected in the water HOH-bending modes where a strong IR band centred around 1660 cm¹ is found. Such a band reflects the strong hydrogen bonding of the water molecules to the phosphate anions in adjacent layers. Spectra show three distinct OHbending bands from strongly hydrogen-bonded, weakly hydrogen bonded water and non-hydrogen bonded water. The Raman phosphate PO-stretching region shows strong similarity between the three minerals. In the IR spectra, complexity exists with multiple antisymmetric stretching vibrations observed, due to the reduced tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Strong IR bands around 800 cm 1 are attributed to water librational modes. The spectra of the three minerals display similarities due to their compositions and crystal structures, but sufficient subtle differences exist for the spectra to be useful in distinguishing the species.

Keywords: BARICITE; BOBIERRITE; VIVIANITE; IRON; PHOSPHATE; RAMAN SPECTROSCOPY; INFRARED; SPECTROSCOPY.

Document Type: Research article

DOI: 10.1180/0026461026660077

Raman and infrared spectroscopic study of the vivianite-group phosphates vivianite, baricite and bobierrite