Pharmacosiderite

Pharmacosiderite is a hydrated basic ferric arsenate, with chemical formula KFe₄(AsO₄)₃(OH)₄·(6-7)H₂O and a molecular weight of 873.38 g/mol. It consists of the elements arsenic, iron, hydrogen, potassium, sodium and oxygen. It has a Mohs hardness of 2 to 3, about that of a finger nail. Its specific gravity is about 2.7 to 2.9, has indistinct cleavage, and is usually transparent or translucent. It has a yellow or white streak and a yellow, green, brown or red color. Its lustre is adamantine, vitreous and resinous, and it has conchoidal, brittle and sectile fracture.

Pharmacosiderite
General
Category	Arsenate minerals
Formula (repeating unit)	KFe4(AsO4)3(OH)4·(6-7)H2O
Strunz classification	8.DK.10
Crystal system	Isometric
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	P43m
Identification
Formula mass	873.38 g/mol
Color	Greens, browns, yellows
Crystal habit	Massive to Crystalline
Cleavage	Distinct on the [100]
Fracture	Brittle
Mohs scale hardness	2.5
Luster	Adamantine
Streak	yellowish green
Diaphaneity	Subtransparent to subtranslucent
Density	2.9 g/cm3
Ultraviolet fluorescence	None
References	[1][2][3]

Pharmacosiderite has an isometric crystal system, with yellowish-green, sharply defined cube crystals. Its crystals are doubly refracting, and exhibit a banded structure in polarized light. When placed in ammonium 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 zones of ore deposits. The alteration of arsenopyrite, tennantite and other primary arsenates can form pharmacosiderite. It can also form from precipitation of hydrothermal solutions, but only rarely. It can be found in abundance in Cornwall, Hungary and the U.S. state of Utah. 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. Siderite, a carbonate mineral, only shares the common element iron with pharmacosiderite.