Feodosiyite
A very rare copper-magnesium chloride hydrate with a unique structure, from the fumarole fields of Tolbachik volcano.
Intro adapted from Wikipedia (CC BY-SA).
Feodosiyite is an IMA-recognized mineral species in the halide class, with the chemical formula Cu₁₁Mg₂Cl₁₈(OH)₈·16H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: Mohs hardness 3; specific gravity 2.57; Light green streak; Vitreous luster; optically biaxial.
IMA symbol Fdy, IMA status: approved, named in 2015.
Occurs as sublimates on basaltic scoria near a volcanic fumarole vent (~100 flC.).
It is found associated with Belloite, avdoninite, sylvite, carnallite, chlorothionite, dioskouriite.
The mineral was named to honor Feodosiy Nikolaevich Chernyshev (1856-1914), Russian geologist, Academician of the Russian Academy of Sciences and Director of Russian Geological Committee (1903-1914).