Vivianite

Vivianite Fe3(PO4)2·8(H2O), hydrated iron phosphate, is a secondary mineral found in a number of geological environments. Usually found as deep blue to deep bluish green prismatic to flattened crystals, most crystals rather small to microscopic, larger ones are rare.It is formed by the alteration of ore deposits near the surface, or of primary phosphates in pegmatites. Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone.Vivianite darkens upon exposure to light, and was named in 1817 after J.G. Vivian, an  English mineralogist who first discovered crystals of the mineral in Cornwall.
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n. vivianite (Minerals)

(n.)
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.
  

General Information:
 Chemical Formula:
Fe++3(PO4)2·8(H2O)
 Composition:
(Molecular Weight = 501.61 gm)
Iron 33.40 % Fe
Phosphorus 12.35 % P 
Hydrogen 3.22 % H 
Oxygen 51.03 % O 
 Empirical Formula:
Fe3+3(PO4)2·8(H2O)
 Environment:
Common secondary mineral in the gossan of metallic ore deposits, pegmatates, sedimentary authigenic mineralization in association with organic materials.
 IMA Status:
IMA Approved
 Name Origin:
Named after the English mineralogist, J. G. Vivian.
Physical Properties:
 Cleavage:
[010] Perfect
 Color:
colorless, green, blue, dark green, or dark bluish green.
 Density:
2.6 - 2.7, Average = 2.65
 Diaphaniety:
Transparent to translucent to opaque
 Habits:
Divergent - Crystals radiate from a center without producing stellar forms., Concretionary - Rounded massive fine-grained materials., Earthy - Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite).
 Hardness:
1.5-2 - Talc-Gypsum
 Luminescence:
None.
 Luster:
Vitreous - Pearly
 Streak:
bluish white
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