The Vivianite Group of minerals are a group of phosphates that have very similar structures. Named after one of their more common members, the group is generally very colorful. All members of this group can be very brilliantly colored. Vivianite is famous for its bluish-green color. Erythrite is famous for its red-purple color. And annabergite for its apple-green color.
The general formula for the group is X3(AO4)2-8(H2O). The X can be a divalent (+2) metal ion such as cobalt, nickel, zinc, iron, magnesium or manganese. The A can be either phosphorous or arsenic. The structure is composed of layers of the AO4 tetrahedrons bonded to the X(O, H2O)6 octahedrons. the bonding between layers is weak and produces the mica-like cleavage.
These minerals are some of the more common minerals of the Vivianite Group: