Haidingerite is a calcium mineral with formula Ca(AsO3OH)·H2O. It crystallizes in the orthorhombic crystal system as short prismatic to equant crystals. It typically occurs as scaly, botryoidal It is soft, Mohs hardness of 2.95. It has refractive indices of nα = 1.590, nβ = 1.602 and nγ = 1.638.
It was originally discovered in 1827 in Jáchymov, Czech Republic mineralogist Wilhelm Karl Ritter von Haidinger It occurs as a dehydration product of pharmacolite , Nevada