Vivianite group

thumb|alt=text|Vivianite from South Dakota, US thumb|alt=text|Vivianite and childrenite from the [[Siglo XX mine (tin mine in Bolivia)]] thumb|alt=text|Vivianite from Bavaria (Germany) thumb|alt=text|Vivianite and albite from Brazil

thumb|Erythrite (cobalt flower) - Bou Azzer, Atlas Mountains, Morocco. Erythrite, also known as red cobalt, previously cobalt ochre is a secondary hydrated cobalt arsenate mineral with the formula . Erythrite and annabergite, chemical formula , or nickel arsenate form a complete series with the general formula . thumb|left|Erythrite crystals

Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate. It is considered a member of the vivianite group and known for its ability to form crystals in a characteristic apple-green color.

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 (18241892), a German chemist from Schneeberg, Saxony, who made the first chemical analysis of the mineral. It has the formula 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 ; cobalt forms the distinctiv

Hörnesite is an arsenate mineral bearing magnesium. It is a member of the vivianite group of minerals, and constitutes the magnesium endpoint of a mineral series with nickel or cobalt replacing the magnesium, ending in annabergite or Erythrite.