Vivianite
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Vivianite is a hydrated iron phosphate mineral found in a number of geological environments. Small amounts of manganese Mn²⁺, magnesium Mg²⁺, and calcium Ca²⁺ may substitute for iron Fe²⁺ in the structure. Wikipedia
2V angle: Measured: 63° to 83.5°, Calculated: 78° to 88°
Color: Colorless, very pale green, becoming dark blue, dark greenish blue, indigo-blue, then black with oxidation
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal system: Monoclinic
Formula mass: 501.61 g/mol
Formula (repeating unit): Fe2+; 3(PO; 4); 2·8H; 2O
Fracture: Fibrous