Wikipedia
Köttigite is a rare hydrated zinc arsenate which was discovered in 1849 and named by James Dwight Dana in 1850 in honour of Otto Friedrich Köttig (1824 - 1892), a German chemist from Schneeberg, Saxony, who made the first chemical analysis of the mineral. It has the formula Zn(AsO)·8HO and it is a dimorph of metaköttigite, which means that the two minerals have the same formula, but a different structure: köttigite is monoclinic and metaköttigite is triclinic. There are several minerals with similar formulae but with other cations in place of the zinc. Iron forms parasymplesite Fe(AsO)·8HO; cobalt forms the distinctively coloured pinkish purple mineral erythrite Co(AsO)·8HO and nickel forms annabergite Ni(AsO)·8HO. Köttigite forms series with all three of these minerals and they are all members of the vivianite group.
The Vivianite Group is a group of monoclinic phosphates and arsenates with divalent cations. The group members are annabergite, arupite, babanekite, baricite, erythrite, hornesite, kottingite, manganhornesite, pakhomovskyite, parasymplesite and vivianite.