Pharmacosiderite is a hydrated basic ferric arsenate, with the chemical formula KFe4(AsO4)3(OH)4·(6-7)H2O and a molecular weight of 873.38 g/mol.

Its lustre is adamantine, vitreous and resinous, and it has conchoidal, brittle and sectile fracture.

Its crystals are doubly refracting, and exhibit a banded structure in polarized light.

This secondary origin mineral is normally formed in the oxidation zones of ore deposits.

The present name, given by J. F. L. Hausmann in 1813, is made up of the Greek words for arsenic and iron, the two most significant consisting elements.