Vivianite

Vivianite (Fe2+Fe2+2(PO4)2·8H2O) is a hydrated iron phosphate mineral found in a number of geological environments. Small amounts of manganese Mn2+, magnesium Mg and calcium Ca may substitute for iron Fe2+ in the structure. Pure vivianite is colorless, but the mineral oxidizes very easily, changing the color, and it is usually found as deep blue to deep bluish green prismatic to flattened crystals.Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone. Vivianite (Fe2+Fe2+2(PO4)2·8H2O) is a hydrated iron phosphate mineral found in a number of geological environments. Small amounts of manganese Mn2+, magnesium Mg and calcium Ca may substitute for iron Fe2+ in the structure. Pure vivianite is colorless, but the mineral oxidizes very easily, changing the color, and it is usually found as deep blue to deep bluish green prismatic to flattened crystals.Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone. It was named by Abraham Gottlob Werner in 1817, the year of his death, after either John Henry Vivian (1785–1855), a Welsh-Cornish politician, mine owner and mineralogist living in Truro, Cornwall, England, or by Jeffrey G. Vivian, an English mineralogist. Vivianite was discovered at Wheal Kind, in St Agnes, Cornwall. Vivianite group minerals have the general formula A3(XO4)2·8H2O, where A is a divalent transition metal cation and X is either phosphorus or arsenic, and they are monoclinic. Group members are: In pure end member vivianite all the iron is divalent, Fe2+, but there are two distinct sites in the structure that these ions can occupy. In the first site the Fe2+ is surrounded by four water molecules and two oxygens, making an octahedral group. In the second site the Fe2+ is surrounded by two water molecules and four oxygens, again making an octahedral group. The oxygens are part of the phosphate groups (PO4)−3, that are tetrahedral. The vivianite structure has chains of these octahedra and tetrahedra that form sheets perpendicular to the a crystal axis. The sheets are held together by weak bonds, and that accounts for the perfect cleavage between them.The crystals are monoclinic, class 2/m, space group C 2/m, with two formula units per unit cell (Z = 2). The approximate values of the unit cell parameters are