BAYLDONITE

    Class : Phosphates, arsenates, vanadates
    Subclass : Hydrated arsenates
    Crystal System : Monoclinic
    Chemistry : Cu3Pb(AsO4)2(OH)2
    Rarity : Fairly common

Bayldonite is secondary arsenate from the oxidation zones of copper deposits where it associates with many other secondary copper and lead minerals (malachite, azurite, cerussite, adamite, olivenite, etc...). Its name was given to it in honor of the physicist John Bayldon who discovered it at the Penberthy mine in Cornwall. It is rare in crystals, the latter not exceeding one centimeter, and usually forms encrustations and hummocky concretions with a fibrous texture, powdery coatings, mimetite or tennantite pseudomorphs. Bayldonite is yellow-green to grass-green, sometimes dark green.

Main photo : Bayldonite from Clara Mine, Germany © Stephan Wolfsried

Bayldonite pseudomorph after mimetite from Penberthy, England © Steve Rust
Bayldonite from Sol Mine, Andalusia, Spain © Angel Romero Escobar
Bayldonite from Clara Mine, Germany © Michael Förch
Bayldonite from Cap Garonne, Var, France © Stephan Wolfsfried

Bayldonite in the World

Bayldonite is very common in the oxidized part of the copper-bearing veins of Cornwall (Gwennap, Saint Day, Gwinear...), but the finest known specimens (5 cm mimetite pseudomorph crystals) come from Tsumeb (Namibia). Other localities include copper deposits in Arizona, including Bisbee, and Broken Hill (Australia).

Bayldonite in France

In France, bayldonite has been harvested in several veins in Beaujolais (Verrière, les Ardillats), Hérault (La Rabasse), Aveyron (Falgayrolles near Villefranches-de-Rouergue) as well as in the Cap Garonne mine (Var).

Twinning and special crystallizations

Twins are known with (-311) as composition plane.

Fakes and treatments

No fake or treatment identified for this mineral species.


Hardness : 4.5
Density : 5.2 to 5.6
Fracture : Irregular
Trace : Green

TP : Translucent to transparent
RI : 1.951 to 1.991
Birefringence : 0.040
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None