Pharmacosiderite is a hydrated basic arsenate K 4)3(OH of 873.38 g/mol. It consists of the elements arsenic, sodium. It has a Mohs hardness of 2 to 3, about that of a finger nail is about 2.7 to 2.9, has indistinct cleavage , and is usually transparent. It has a yellow and a yellow, green. Its lustre is adamantine and resinous, and it has conchoidal fracture.
Pharmacosiderite has an isometric crystal system , with yellowish-green, sharply defined cube. Its crystals are doubly refracting , and exhibit a banded structure in polarized light solution , a crystal changes color to a distinguishing red. Upon placing it into dilute hydrochloric acid the original color is restored. This secondary origin mineral is normally formed in the oxidation deposits. The alteration of arsenopyrite and other primary arsenates can form pharmacosiderite. It can also form from precipitation solutions, but only rarely. It can be found in abundance in Cornwall , Hungary. When it was first discovered, pharmacosiderite was known as cube ore. 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. Pharmakos means poison, which is related to arsenic, and sideros means iron. Pharmacolite and picropharmacolite, which are different arsenates, are not associated besides via nomenclature , a carbonate mineral, only shares the common element iron with pharmacosiderite.