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Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Basic hydrated potassium iron arsenate
Chemical Formula: KFe3+4(AsO4)3(OH)4.6-7H2O
Method(s) of Verification: Gwaith-yr-Afon Mine - XRD (Natural History Museum, London).

Chemical Group:

Geological Context:

A colourless, cubic, microcrystal of pharmacosiderite from Gwaith-yr-Afon Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: pharmacosiderite is typically found, along with scorodite, as an oxidation product of arsenopyrite. It may also, however, develop during oxidation of other iron-bearing sulphides such as pyrite or chalcopyrite in the presence of arsenate ions produced by the decomposition of other, less common primary arsenic-bearing phases. Where well-crystallized, pharmacosiderite is distinctive among the supergene minerals due to its typically well-formed cubic crystals, brownish to greenish colour and association with primary As-bearing phases, as in occurrences in Cornwall from which it is well-known.
Occurrence in Wales: despite the widespread occurrence of arsenopyrite in Wales, pharmacosiderite is itself very rare, being restricted to a single occurrence in Central Wales where the primary arsenic source was minor Co-Ni sulpharsenides associated with base-metal sulphides (Rust & Mason, 1994).

Key Localities:


  1. Rust, S.A. & Mason, J.S., 1994. An unusual occurrence of arsenate minerals at Gwaith-yr-Afon mine, Dyfed, Wales. Journal of the Russell Society, 5(2), 109-113.