Abstract The presence of a magnesian vivianite (Fe 2+ ) 2.5 (Mg,Mn,Ca) 0.5 (PO 4 ) 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 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 Fe 2+ 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.