Identification by RAMAN Microscopy of magnesian vivianite formed from Fe2+, Mg, Mn2+ and PO43- in a Roman camp near fort Vechten, Utrecht, The Netherlands

The presence of a magnesian vivianite (Fe (2+))(2.5)(Mg,Mn,Ca)(0.5)(PO4)(2).8H(2)O, has been identified in a soil sample from a Roman camp near Fort Vechten, The Netherlands, using a combination of Raman microscopy and scanning electron microscopy. An unsubstituted vivianite and baricite were characterised for comparative reasons. The split phosphate-stretching mode is recognised around 1115, 1062 and 10 15 cm(-1), while the corresponding bending modes are found around 591, 519, 471 and 422 cm(-1). The substitution of Mg and Mn for Fe2+ in the crystal structure causes a shift towards higher wavenumbers compared to pure vivianite. As shown by the baricite sample substitution causes a broadening of the bands. The observed broadening however is larger than can be explained by substitution alone. The low intensity of the water bands, especially in the OH-stretching region between 2700 and 3700 cm(-1) indicates that the magnesian vivianite is partially dehydrated, which explains the much larger broadening than the observed broadening caused by substitution of Mg and Mn in vivianite and baricite.