Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper lead arsenate hydroxide
Chemical Formula: Cu3Pb(AsO4)2(OH)2
Method(s) of Verification: Gwaith-yr-Afon Mine: XRD at the National Museum of Wales (no. NMW X-941); Dolyhir Quarry: XRD and EDS at Manchester Museum (MANCH:XRD1115).

Chemical Group:

  • Arsenates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Pale apple-green bayldonite crust lining a cavity in quartz. Gwaith-yr-Afon Mine. Field of view 4 mm across. National Museum of Wales Collection, no. NMW 2003.1G.M.270. Photo D.I. Green. © National Museum of Wales.
Pale green bayldonite from Sigenlas Mine in the Central Wales Orefield. Specimen and photo S.A. Rust. © S.A. Rust.
Introduction: bayldonite is a supergene mineral, typically found in the oxidized parts of veins and other ore deposits in which the primary mineralogy includes copper, lead and arsenic-bearing minerals. It is typically associated with a range of other supergene arsenates, such as mimetite.
Occurrence in Wales: despite the frequency with which lead-copper-arsenic (Pb-Cu-As) bearing primary mineral associations occur in Wales (e.g. Parys Mountain, Snowdon Caldera veins, Dolgellau Gold-belt), bayldonite is quite restricted in its occurrence. Thin crusts of bayldonite have been identified at a couple of mines in the Central Wales Orefield where very minor primary As-bearing minerals occur in a Pb-Cu dominated primary assemblage and traces have been detected at Dolyhir Quarry in the Welsh borders (Cotterell et al., 2011).

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: Cotterell et al. (2011) describe green glassy bayldonite encrusting pale yellow powdery gartrellite on a single specimen found at Dolyhir Quarry. Part of the specimen (collected by A. Dean) is accessioned as NMW 2006.16G.M.5 in the mineral collection at the National Museum of Wales.
  • Gwaith-yr-Afon Mine, Goginan, Ceredigion: at this locality, where a number of arsenate-bearing supergene minerals have been recorded underground (Rust & Mason, 1994), a single block of quartz carrying bayldonite was found on the tips in 1994. The bayldonite occurred as a thin, characteristically apple-green, film on a fracture-surface in the quartz.
  • Sigenlas Mine, Llanidloes, Powys: bright green crusts of bayldonite associated with mimetite microcrystals occur on a number of specimens collected by J.S. Mason and now in the mineral collection at the National Museum of Wales (see for example NMW 95.55G.M.248).


  1. Cotterell, T.F., Green, D.I., Hubbard, N., Mason, J.S., Starkey, R.E. and Tindle, A.G., 2011. The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales. UK Journal of Mines and Minerals, 32, 5-61.
  2. 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.