What is "vivianite"

Vivianite \Viv"i*an*ite\, n. [So called by Werner after the English mineralogist F. G. Vivian.] (Min.) A hydrous phosphate of iron of a blue to green color, growing darker on exposure. It occurs in monoclinic crystals, also fibrous, massive, and earthy.

n. (context mineralogy English) A blue, green or colourless mineral of monoclinic crystals, iron₃(phosphorusoxygen₄)₂'''·'''8water.

Vivianite is a hydrated iron phosphate mineral found in a number of geological environments. Small amounts of manganese Mn, magnesium Mg and calcium Ca may substitute for iron Fe in the structure. Pure fresh 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 John Henry Vivian (1785–1855), a Welsh-Cornish politician, mine owner and mineralogist living in Truro, Cornwall, England. John Vivian discovered the mineral at Wheal Kind, in St Agnes, Cornwall.