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:

  • Arsenates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
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:

  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: pharmacosiderite occurs as small (0.25 mm) cubic crystals in thin drusy crusts on goethite and quartz in proximity to heavily corroded chalcopyrite. Probable scorodite is the only other associated supergene mineral (Rust & Mason, 1994).


  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.