A kind of pale-blue substance was found on ancient textiles excavated from the Hejiazao Song Tomb in Hengyang, Hunan Province. In this study, we used a combination of SEM-EDS, LRS, XRD, XRF and OM for analysis. The results show that the substance was a ferrous phosphate mineral—vivianite [Fe₃(PO₄)₂·8H₂O], which is mostly formed under a reducing and sulfur-deficient but iron and phosphorus-rich environment. Microbial activities also affect its formation. The discovery of this mineral is instructive for studying the preservation state and environmental characteristics of the tomb. This mineral may be used to judge special burial conditions of archaeological remains.