Hörnesite (Neuenberg, Sainte-Marie-aux-Mines, Haut-Rhin). Musée de minéralogie de Strasbourg — Photo: Ji-Elle · CC BY-SA 3.0
Hörnesite
The magnesium member of the vivianite group — a hydrated magnesium arsenate in series toward annabergite and erythrite.
Intro adapted from Wikipedia (CC BY-SA).
Hörnesite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Mg₃(AsO₄)₂·8H₂O.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Vivianite structural group.
Key properties: Mohs hardness 1; specific gravity 2.57–2.73; Resinous to waxy, pearly on cleavages luster; optically biaxial.
IMA symbol Hns, grandfathered status — known before the IMA’s founding, named in 1860.
Found in limestone blocks, thermally metamorphosed, in a volcanic tuff (Fiano, Italy).
It is found associated with Fluoborite, fluorite, hydromagnesite (Fiano, Italy).
The mineral was named to honor Moritz Hörnes (1815–1868), Curator of the Imperial Cabinet, Vienna, Austria.