Raman spectroscopic study of vivianites of different origins
Frost, Ray L.; Weier, Matt
published: Sep 27, 2004
Raman spectroscopy has been used to study a selection of vivianites from different origins. A band is identified at around 3480 cm-1 whose intensity is sample dependent. The band is attributed to the stretching vibration of Fe3+ OH units which are formed through the autooxidation of the vivianite minerals either by self-oxidation or by photocatalytic oxidation according to the reaction:(Fe2+)3(PO4)2Â·8H2O + 1/2O2 â (Fe2+)3Â x(Fe3+)x(PO4)2(OH)xÂ·(8Âx)H2O in which some of the water of crystallization is converted to hydroxyl anions. Complexity of the OH stretching region through the overlap of broad bands is reflected in the water HOH deformation modes at 1660 cmÂ1. Using the infrared bands at 3281, 3105 and 3025 cmÂ1, hydrogen bond distances of 2.734(5), 2.675(2) and 2.655(2) Ã are calculated. Vivianites are characterised by an intense band at 950 cmÂ1 assigned to the PO4 symmetric stretching vibration. Low Raman intensity bands are observed at ~1077, ~1050, 1015 and ~ 985 cmÂ1 assigned to the phosphate PO4 antisymmetric stretching vibrations. Multiple antisymmetric stretching vibrations are due to the reduced tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Two bands are observed at ~ 423 and ~ 456 cmÂ1 assigned to theν2bending modes. For the vivianites four bands are observed at ~ 584, ~ 571, ~ 545 and ~ 525 cmÂ1 assigned to the ν4modes of vivianite.