Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Copper yttrium calcium arsenate hydroxide hydrate
Chemical Formula: (Y,Ca)Cu6(AsO4)3(OH)6.3H2O
Method(s) of Verification: Gwaith-yr-Afon Mine - XRD & EDX (National Museum of Wales).

Chemical Group:

  • Arsenates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Agardite-(Y) forming small (<2 mm) clusters of acicular crystals of typical green to white colour, on a matrix of earthy Mn oxides. Specimen from Gwaith-yr-Afon Mine. © Steve Rust.
Scanning electron micrograph of acicular 'bow-tie' sprays of agardite-(Y) from Gwaith-yr-Afon Mine. Scale bar 100 microns (0.1 mm). National Museum of Wales specimen (NMW 2003.1G.M.275). © National Museum of Wales.
Introduction: agardite is a rare mineral restricted in its occurrences to supergene alteration zones overlying polymetallic ore deposits in which arsenic is present. A wide range of other supergene minerals may be associated. It is related to, and resembles, mixite - in which bismuth takes the place of the rare earths. It also resembles finely fibrous pale malachite, and in consequence is not a particularly easy mineral to identify without using analytical techniques.
Occurrence in Wales: agardite-(Y) is only known from three UK localities - one in Wales and two in Cornwall. The Welsh occurrence is at Gwaith-yr-Afon Mine in Central Wales, where it occurs as a member of an unusually diverse arsenate assemblage in an area not generally known for its arsenic minerals (Rust & Mason, 1994).

Key Localities:

  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: agardite-(Y) occurs rarely as tufts of minute (<0.25 mm) but lustrous deep to pale green acicular crystals on joints in mudstone breccia which are coated in friable black Fe-Mn oxides. Associated minerals are malachite and mimetite. The few specimens known were collected underground in the early 1990s.


  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.