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From The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]