The presence of a magnesian vivianite (Fe2+)2.5(Mg, Mn, Ca)0.5(PO4)2. 8H2O, has been identified in a soil sample form a Roman camp near Fort Vechten, The Netherlands, using a combination of Raman microscopy and scanning electron microscopy. An unsubstituted vivianite and baricite were characterized for comparative reasons. The split phosphate-stretching mode is recognized around 1115, 1062 and 1015 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 baričite 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 baričite.