Haidingerite is a calcium arsenate mineral with formula Ca(AsO3OH)·H2O. It crystallizes in the orthorhombic crystal system as short prismatic to equantcrystals. It typically occurs as scaly, botryoidal or fibrous coatings. It is soft, Mohs hardness of 2 to 2.5, and has a specific gravity of 2.95. It has refractive indices of nα = 1.590, nβ = 1.602 and nγ = 1.638.