Geigerite is a mineral, a complex hydrous manganese arsenate with formula: Mn5(AsO3OH)2(AsO4)2·10H2O. It forms triclinic pinacoidal vitreous colorless, red to brown crystals. It has a Mohs hardness of 3 and a specific gravity of 3.05.
It was discovered in Grischun, Switzerland in 1989, it is apparently named after the mineral collector Richard M Geiger. It is also found in Liguria, Italy.