Vivianite, Iron Phosphate,
Formula Fe3(PO4)2.8H2O

Crystal system: Monoclinic.
Hardness: 1.5 to 2
Density: 2.7 gms/cm3
Colour: Colourless initially, but darkens to green, blue-green then black because of partial oxydation from ferrous (Fe2+) to ferric (Fe3+).
Streak: Colourless.

Vivianite occurs as an alteration product of iron ores and iron phosphates. The finest crystalline specimens of Vivianite are found in Bolivia.

This nice transparent, green specimen of Vivianite comes from the Potosi Department of Bolivia.

Vivianite from the Potosi Department of Bolivia