Mineral Species: Pharmacosiderite

Chemical Formula: KFe4(AsO4(OH)4 - 7H2O

Chemical Name: Hydrated Potassium Iron Arsenate Hydroxide

Mineral Class: Arsenate

Mineral Notes: Pharmacosiderite is a rare secondary arsenate mineral. It typically forms in the oxidation zones of iron ore deposits from the alteration of Arsenopyrite, Tennantite and other primary arsenic minerals, and less often, directly from precipitation of hydrothermal solutions. The mineral forms cubic crystals. This is extremely unusual for an arsenate or even a phosphate. Occasionally, Pharmacosiderite can be confused with Fluorite but the latter has perfect cleavage and is harder. The mineral should not be confused with Siderite, which is a simple iron carbonate. Pharmacosiderite lends its name to a small mineral group called the Pharmacosiderite Group. These are a set of closely related arsenates of which pharmacosiderite is the most common member. Others in the group include Alumopharmacosiderite and Sodiumpharmacosiderite.