Abstract
The molecular structure of the three vivianite-structure, compositionally
related phosphate minerals vivianite, baricite and bobierrite of formula
M32+(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
hydroxyl-stretching region are complex with overlapping broad bands. Hydroxyl
stretching vibrations are identified at 3460, 3281, 3104 and 3012
cm−1 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
cm−1 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 OH-bending bands fromstrongly 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.
The intramolecular hydrogen bonded–halogen bond: a new strategy for preorganization and enhanced binding
