Infrared and Raman spectroscopic characterization of the phosphate mineral fairfieldite Ca2(Mn2+,Fe2+)2(PO4)2 2(H2O).

Abstract

Raman spectroscopy complimented with infrared spectroscopy has been used to determine the molecular structure of the phosphate mineral fairfieldite. The Raman phosphate ðPO4Þ3_ stretching region shows strong differences between the fairfieldite phosphate minerals which is attributed to the cation substitution for calcium in the structure. In the infrared spectra complexity exists with multiple ðPO4Þ2_ antisymmetric stretching vibrations observed, indicating a reduction of the tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Strong Raman bands around 600 cm_1 are assigned to m4 phosphate bending modes. Multiple bands in the 400–450 cm_1 region assigned to m2 phosphate bending modes provide further evidence of symmetry reduction of the phosphate anion. Three broadbands for fairfieldite are found at 3040, 3139 and 3271 cm_1 and are assigned to OH stretching bands. By using a Libowitzky empirical equation hydrogen bond distances of 2.658 and 2.730 Å are estimated. Vibrational spectroscopy enables aspects of the molecular structure of the fairfieldite to be ascertained.