Formula: Cu3PbO(AsO3OH)2(OH)2
Anhydrous arsenate containing hydroxyl
Specific gravity: 5.5
Hardness: 4½
Streak: Siskin green to apple green
Colour: Green, apple-green, yellow-green
Solubility: Soluble with difficulty in hydrochloric acid

Hydrothermal environments

Bayldonite is a relatively rare secondary mineral occurring in the oxidised zones of copper and lead bearing deposits. Associated minerals include adamite, anglesite, azurite, baryte, beudantite, cerussite, duftite, keyite, malachite, mimetite, olivenite, schultenite and tsumebite (Mindat).


At the Kintore open cut, Broken Hill, New South Wales, Australia, bayldonite most commonly occurs with mimetite, segnitite - beudantite, mawbyite and members of the olivenite - adamite series. It also occurs with carminite, duftite and conichalcite, and with late-stage minerals including brochantite, malachite, azurite and iodargyrite. In one unusual assemblage bayldonite crystals were found with pyromorphite and natrojarosite on gahnite-bearing quartzite (AJM 3.1.55).

At the New Cobar deposit, New South Wales, Australia, bayldonite occurs as crystals on quartz commonly associated with duftite. Other associated minerals include gartrellite, segnitite, agardite-(Y) and azurite; a bayldonite pseudomorph after mimetite has also been found here (AJM 11.2.79).

At Kayrakty, Kazakhstan, bayldonite occurs in the oxidation zone of a metasomatic lead-zinc deposit in the upper layers of baryte lenses of considerable size embedded in sandstone. Bayldonite is formed by pseudomorphic substitution of cerussite, resulting in accumulations in malachite and azurite. Occasionally bayldonite is found on thick crusts formed in fractures in the baryte. The bayldonite is usually intergrown with beudantite, and sometimes has cavities with small crystals of anglesite and azurite (AM 66.148–153).
Associated minerals include beudantite, anglesite, cerussite, malachite, azurite and baryte (HOM).

At Tsumeb, Namibia, bayldonite is associated with mimetite, olivenite, azurite, malachite, tsumebite, cuprian adamite, duftite, keyite, schultenite and philipsbornite (HOM). Bayldonite pseudomorphs after mimetite have been found here (KL p199).

At the type locality, Penberthy Croft mine, St Hilary, Cornwall, England, UK, bayldonite is associated with quartz (Mindat).

At Brandy Gill mine, Caldbeck, Allerdale, Cumbria, England, UK, bayldonite occurs with galena on a quartz matrix (AESS).

At the San Rafael mine, Nye county, Nevada, USA, spherules and druses to 1 mm of bayldonite have been found associated with plumbojarosite and wulfenite crystals. Rarer single crystals of chlorargyrite - bromargyrite and coatings of arseniosiderite also occur in this assemblage (R&M 85.6.516).

The Activity-pH diagram below was calculated at 298.2 K for the main Cu2+ and Pb2+ arsenate minerals. Boundaries are calculated for constant activities (roughly equivalent to concentrations) of Pb2+ and Cl- ions in solution, over a range of values of pH and of Cu2+ activity. Higher chloride activites make the stability fields of bayldonite and duftite vanish, with respect to the encroachment of that of mimetite (LMW p269).
stability Cu Pb.jpg

The arsenate mineral formulae are:
philipsbornite PbAl3(AsO4)(AsO3OH)(OH)6
mimetite Pb5(AsO4)3Cl
duftite PbCu(AsO4)(OH)
bayldonite Cu3PbO(AsO3OH)2(OH)2
olivenite Cu2(AsO4)(OH)
cornwallite Cu5(AsO4)2(OH)4
clinoclase Cu3(AsO4)(OH)3

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