Geigerite
A complex hydrous manganese arsenate forming vitreous colorless-to-brown triclinic crystals.
Intro adapted from Wikipedia (CC BY-SA).
Geigerite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Mn²⁺₅(AsO₄)₂(AsO₃OH)₂·10H₂O.
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Lindackerite structural group.
Key properties: Mohs hardness 3; specific gravity 3.05; White streak; Vitreous to pearly luster; refractive index 1.63; optically biaxial.
IMA symbol Ggr, IMA status: approved, named in 1985.
Found in cavities in radiolarites, formed by metamorphism of manganese oxide ores.
It is found associated with Brandtite, sarkinite, grischunite, bergslagite, manganberzeliite, tilasite.
In honor of Dr. Thomas Geiger (1886–1976), Wiesendangen, Switzerland, who studied Falotta manganese ores.