vivianite, phosphate mineral, hydrated iron phosphate [Fe3(PO4)2·8H2O], that occurs as colourless when freshly exposed. After exposure to air, the iron oxidizes and the mineral’s colour becomes light green, light blue, blue-green, dark green, dark blue, or black, depending on the length of exposure. Vivianite appears as glassy crystals in the weathered zones of phosphate deposits and as concretions in clays (widespread). It also occurs in recent sediments, in lignite and peat, in forest soils, and as the colouring agent in odontolite (bone turquoise). Localities of occurrence include England, Germany, Bolivia, Japan, and Ukraine. It is abundant in New Jersey and Colorado. For detailed physical properties, see phosphate mineral (table).