Haidingerite is a calcium arsenate mineral with formula Ca(AsO<sub>3</sub>OH)÷H<sub>2</sub>O. It crystallizes in the orthorhombic crystal system as short prismatic to equant crystals. 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 nñ = 1.590, nò = 1.602 and nó = 1.638.
It was originally discovered in 1827 in Jáchymov, Czech Republic. It was named to honor Austrian mineralogist Wilhelm Karl Ritter von Haidinger (1795âÂÂ1871). It occurs as a dehydration product of pharmacolite in the Getchell Mine, Nevada.