Bayldonite

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).

References:

  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.